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Nucleolar and chromatin cycles in abies: microsporogenesis in Abies grandis with particular reference… Rattenbury, John Alban 1945

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ITUCISOIAR AMD CHROMATID CYOIES H ABIES ' Microsporogenesis in-- Abies grandis with p a r t i c u l a r reference to the structure and development of the nucleolus and. the t r a n s f e r of nucleolar materia,! to the chromosomes. Submitted i n p a r t i a l f u l f i l m e n t of c r e d i t towards a major i n Biology and Botany f o r the degree of Master of A r t s . The U n i v e r s i t y of B r i t i s h Columbia John Alban Rattenbury. October, 1945. TABLE Off OOHTMIS I. Introduction. . .page i 1. O r i g i n of Problem. i 2. Acknowledgements... i 3. Statement of Problem*. i 4. Discussion of Problem* i i I I . M a t e r i a l s and Methods..........s..............» 1 1. Source of .Materials. 1 2. Times of C o l l e c t i o n 1 5. Method of Preservation. 1 4. Embedding, Sectioning and Staining Procedures 1 5. Drawings and Photographs®.................... 2 6. Measurements................................. 2 I I I . D e s c r i p t i o n of Microsporogenesis. 5 T¥» Nucleolar Development and Morphology... .«« 4 V. i-Tiicieolar-Ohromosome Relations 6 ¥1. Size R e l a t i o n s h i p s of O e l l P a r t s . . . . . . . . . . . , . . • 7 V I I . Conclusion. 10 V I I I . References. I v IX. P l a t e s . ... v i . gUOLEOIAR ASP' OHBOM&CT CYCLES IN ABIES I. Introduction 1. O r i g i n of Problem The problem o r i g i n a t e d as part of a study of the genetics of economic trees being c a r r i e d out under the d i r e c t i o n of Dr. A.H.Hutchinson of the Department of Biology and Botany at the U n i v e r s i t y of B r i t i s h Columbia. The extent of, the present research covers one phase,, i n p a r t , of the reproductive cycles i n Abies. Other phases, have been previously studied and w i l l - b e studied at a l a t e r date. 2. Acknowledgements I should l i k e to'acknowledge the assistance, guidance, and. encouragement received from Dr. Hutchinson which have made t h i s research p o s s i b l e . . / I should l i k e to acknowledge the s t a t i s t i c a l assistance received from Dr. V.O.Brink of the Department-of Agronomy i n . t h i s U n i v e r s i t y . I should l i k e to acknowledge the f i n a n c i a l assistance , received from the U n i v e r s i t y of B r i t i s h Columbia under a grant for' 'Genetics of Economic Trees'*' S« Statement of Problem E x c e l l e n t preparations were obtained of m a t e r i a l of Abies grandis Lindley undergoing microsporogenesis. These preparations were stained very s p e c i f i c a l l y to show the i i . presence of hoth euchromatln and nucleolar m a t e r i a l , (see under methods). The purpose of t h i s paper i s to i n d i c a t e the ! extent of these materials during the various stages of red-u c t i o n - d i v i s i o n and to. attempt, to harmonize the f i n d i n g s with current theories of chromosome and nucleolar chemistry and physics. With t h i s i n view,...the paper has "been divided into four sections dealing with: ( l ) the extent and character of the stages of r e d u c t i o n - d i v i s i o n ; (2) the morphology and development of the nucleolus; (3) the r e l a t i o n s h i p "between the nucleolus and the chromosomes; (4) the size r e l a t i o n s h i p s of the c e l l p a r t s during the stages before the metaphase* 4. Discussion of Problem The chemistry of the nucleus, i s f a r from being f u l l y understood. Most workers agree that the p r i n c i p a l components of the nucleus are p r o t e i n s of the protamine and histone type plus n u c l e i c a c i d s . Stedman and Stedman, 1943, however pre-sent considerable evidence for the presence of another form of p r o t e i n , which they c a l l .'chromosomin1, c o n s t i t u t i n g from 50-72^ of the nuclear contents. These workers have suggested, with experimental v e r i f i c a t i o n , that the chromosomin i s located i n the chromatin and assume that i t plays an import-. . ant part i n the transmission of hereditary characters. The n u c l e i c acids have been studied i n some d e t a i l , c h i e f l y by means of d i f f e r e n t i a l s t a i n s , u l t r a - v i o l e t absorp-t i o n spectra and enzymatic d e s t r u c t i o n . The basic formula f o r the n u c l e i c acids* (see reviews by ^Painter ,'• 1943; Gates, 1942), i i i . i s as fo l l o w s : Phosphoric a c i d - ribose sugar - adenine (purine) Phosphoric a c i d - rioose sugar - cytosine (pyrimidine) Phosphoric a c i d - ribose sugar - guanine (purine.) Phosphoric a c i d - ribose sugar - u r a c i l or methyl u r a c i l (pyrimidine) The chemical condition of the sugar divides the whole group of n u c l e i c acids into two'main sub-groups: the desoxy* ribose n u c l e i c - a c i d s which c o n s t i t u t e the stainable p o r t i o n of the euchromatin and are known also as thymonueleie asids* and the ribose n u c l e i c acids found both i n the cytoplasm and i n the c e n t r a l o x y p h i l p o r t i o n of the nucleolus'. This l a t t e r group has been c a l l e d the phytonucleie a c i d s . . Schultz, 1941, has given a p i c t u r e of the chemical straicture of both nucleolus and chromosomes which, sums up the present-day knowledge as w e l l as possible... Ag cor ding to him the nucleolus c o n s i s t s of a histone-type of p r o t e i n to which are l i n k e d the ribose n u c l e i c acids t y p i c a l of the species. The chromosome.consists of a*protamine thread with nucleo-p r o t e i n , (combinations of n u c l e i c acids and p r o t e i n s ) , at i n t e r v a l s , the whole being sheathed by a more complex p r o t e i n s i m i l a r to that found i n the nucleolus. The histone-type p r o t e i n sheath is. the. green-staining m a t e r i a l of the l i g h t -green procedure:. Fiirther d i s c u s s i o n of the nature of the materials of the nucleus w i l l be presented as the data are analyzed. . J - « I I . Materials- and Methods 1. Source of Ma t e r i a l s The materials used f o r t h i s research were obtained e n t i r e l y from a s i n g l e tree i n the Bo t a n i c a l Gardens of the un i v e r s i t y of B r i t i s h Columbia. 2. Times of Collection-, The c o l l e c t i o n s were made i n two seasons, 1944 and 1945. The 1944 c o l l e c t i o n was "begun on February 11 and continued u n t i l March 27,.''-reduction-division 'being completed about March 20. The 1945 c o l l e c t i o n began on March .10 and continued u n t i l March 30,. r e d u c t i o n - d i v i s i o n being complete about March :22. C o l l e c t i o n s were made at varying i n t e r v a l s - o f 24 hours to several d.ays. 3. Method of Preservation A l l m a t e r i a l was k i l l e d and f i x e d i n a mixture of 2 parts of absolute a l c o h o l to 1 part of g l a c i a l a c e t i c a c i d . A f t e r thorough washing i n 7.0.% a l c o h o l , the ma t e r i a l was stored i n 70/6 a l c o h o l u n t i l embedded. 4. Embedding, Sectioning and St a i n i n g Procedures The. .material was run up from 70% a l c o h o l and embedded . fo l l o w i n g the schedule below: ^>D/Q Q<iCOl3.0l# « e • • • 0 • • * » • e • a • • • « • • « • • • # 41: ill? S o ' a b s o l u t e a l c o h o l . . . . . . . . . 4: hrs. -absolute alcohol........-'...............^4 hrs. -; b()% absolute a l c o h o l + 20% benzene.....2 hrs. 65% absolute a l c o h o l 4 35,70 benzene..... 2 hrs. 50% absolute aleohoi * 507? benzene.... .2 hrs. 35% absolute a l c o h o l * 65% benzene -2 hr s . 20/6 absolute a l c o h o l + 80% benzene...... 2 hrs. 100% benzene. 2 h r s . 100% benzene.4- wax................overnight. 2. The wax consisted of standard commercial parawax plus about. Ifo of low melting-point wax, (40-42°o) . The embedding was completed as quickly as p o s s i b l e , (4-6 hrs.) to prevent undue shrinkage. Sectioning was done by means of a rotary microtome using a razor-blade. Thickness--o-f sections was 12-15 microns."' The m a t e r i a l was stained with the S'eulgen's and Light Green procedure of Semmens and Bhaduri, 1941. 5. Drawing s and. Pho to graphs The m a t e r i a l was examined by means of a Seibert binocular compound microscope using a n o i l - i m m e r s i o n objective and o i l between s l i d e and condenser. Using the ooculars provided with the microscope i t was possible to obtain magnifications of 700 and 1500 diameters-. Drawings were made v/ith a camera-lucida using, the oculars provided or a s p e c i a l ocular of 50 diameters. 'Photographs were taken with a h o r i z o n t a l camera using^ a tungsten-arc lamp to provide u l t r a - v i o l e t wavelengths: i s o l a t e d hy means of.a Corning #5970 f i l t e r . Corrections were made i n focussing these shorter wave-lengths. Eastman Process p l a t e s were used f o r the photographs. These were developed i n a D-76 j developer. 6. Measurements Diameters of c e l l s * n u c l e i and n u c l e o l i were measured by means of a camera-lucida and d i v i d e r s . Where these structures were not p e r f e c t l y s p h e r i c a l * an average of the major and minor: axes was used. Volumes were determined by the formula f o r the volume of a sphere. g o I I I . D e s c r i p t i o n of Microsporogenesis The early c o l l e c t i o n s may he presumed to he i n the i n t e r -phase or r e s t i n g stage. There i s no i n d i c a t i o n of a sudden appearance of clromonemata, hut rather a gradual increase i n thickness and c o n t i n u i t y * (apparent), of the threads. See Pl a t e I , f i g s . 1 & 2. IJeptotene was thought to "begin about February 26 since the stages of prophase proceeded f a i r l y r a p i d l y from t h i s time on. P I . I , f i g s . 3 & 4. P a i r i n g of homologues and s y n i z e s i s , (zygotene), began about the end of February, (1944 c o l l e c t i o n ) ,;. and continued through-the e a r l y part of March - about two weeks - when the f i r s t thickening of pachytene was observable. P I. I I , f i g s . 5 & 6. At t h i s stage, r e l a t i o n a l c o i l i n g of the homologous chromosomes was detectable and t h i s i s i n d i c a t e d i n f i g . 6 . By the 17th of March, the diplotene chromosomes, could be recognized with d i s t i n c t evidence of four chromatids. P I . I I f i g . 7; P I . I l l , f i g . 11 & 12. R e l a t i o n a l c o i l i n g was s t i l l quite definite.• See P i . I I , f i g . - 8 ; P I . I l l , f i g . 9. By March 20, the f i r s t , ( m e i o t i c ) , d i v i s i o n was complete i n some of the c e l l s , while others were s t i l l at l a t e diakinesiSi.Pl> IV, f i g . . 14 shows the normal miiphase plate seen from polar. view. The. extremely t i g h t r e l a t i o n a l c o i l s were r e a d i l y seen i n the preparations. C o i l i n g appeared c o n s i s t e n t l y to be i n one d i r e c t i o n , I.e. a counter~clockwise s p i r a l proceeding from the proximal to the d i s t a l p o r t i o n . This i s i n accord with the observations, recorded the drawings of P I . I I , f i g . 6 and P I . I l l , f i g * 9, 4. The second, {equational), d i v i s i o n took place two days l a t e r a f t e r a very short interphase during which the nucleolus d i d not appear to he comple&tely reorganized, P I . IY, f i g s . 15, 16, 17 show the f i r s t ; d i v i s i o n telophase, the second d i v i s i o n anaphase, and the second d i v i s i o n telophase with the formation of the quartet, (spore t e t r a d ) . r e s p e c t i v e l y . The much divided state of the nucleolus i s observable i n both telophases previous to i t s r e e o n s t i t u t i o n a f t e r i t disappears during diplotene. See P I . I l l , f i g s . 12 & 13. The large number of chromosqmes, (12 p a i r s ) , i n Abies renders i t a rather poor subject f o r some phases of chromo-some study, and no attempt has been made as yet to analyse the material, f o r abnormal behaviour, or to : i d e n t i f y the chromosomes by t h e i r shape.and s i z e i n t h i s paper. Some evidence of major c o i l s was v i s i b l e :at diplotene; P I . I l l , f i g s . 12-..& 13. D u r i n g l a t e r stages, the nattire of/ f the.preparations was such as to obscure t h i s phenomenon. The preparations d i d not show the,presence of the chromosome. :sheath a f t e r diplotene. ' » t IY. Nucleolar Development and Morphology:. The s p e c i f i c i t y of l i g h t green and some other s t a i n s for the histone component of the nucleolus has rendered t h i s s tructure an i n t e r e s t i n g one f o r the c y t o l o g i s t . The e a r l i e s t c o l l e c t i o n , (February 11, 1944), shows some d i s p a r i t y i n size- and numbers of n u c l e o l i In the d i f f e r e n t c e l l s . Some c e l l s appear to have only, one large nucleolus while- others have as many as four smaller ones. The t o t a l •volume, although."by .no means constant, i s about the same i n a l l c e l l s as i s i n d i c a t e d by the r e l a t i v e l y small amount of standard error of the mean of ten observations. In f a c t the range i s scarcely greater,than that about the mean when only a s i n g l e nucleolus was present i n a l l c e l l s , (March IS, 16 c o l l e c t i o n s ) - . This i s p l o t t e d g r a p h i c a l l y i n PI.IX. The structure of the nucleolus i n these early stages i s : also v a r i a b l e . The outer, ( b a s i p h i l ) , l a y e r , (see McSill,1906 Guenther, 1903;' Cowdry and Kitchen, 1930; Lucas, 1940; Gates, 1942), i s c o n s i s t e n t l y stained green* The inner p o r t i o n , C o x y p h i l ) , consists of one or more gl o b u l a r , granular s t r u c -t u r e s , e i t h e r c o l o r l e s s or pale'pirnr. There seems to be considerable evidence from the preparations that the smaller c e n t r a l portions eventually fuse to form a s i n g l e large inner sphere, (the ribose n u c l e i c a c i d component - Schultz, 1941). This f u s i o n i s reported by Gates, 1942; Zirfele, 1928, 1930. P I . I, f i g s . 1, 2 & 4 show various stages i n the f u s i o n of , the o x y p h i l portions of the nucleolus. I t i s thought that the h a s i p h i l p o r t i o n i s more l i q u i d and that the f u s i o n i s instantaneous. P I . I , f i g . 3 i l l u s t r a t e s a condition found i n many of the''cells--at t h i s stage, (February 26). The two p a r a l l e l l i n e s w i t h i n the c e n t r a l , o x y p h i l part of the nucleolus suggest the presence of powerful e l e c t r i c a l charges f o r c i n g some part of the contents of the inner sphere to opp-o s i t e sides. The p o r t i o n between the l i n e s appears very c l e a r i n the stained m a t e r i a l . PI.V shows photographs of n u c l e o l i at various stages of development. 6. V. Nucleolar-Chromosome fielations I t i s now generally accepted that the contents, i n par t , of the nucleolus form the sheath of the chromosomes when the former disappears at the end of diplotene. {Schultz, 1941; Gates, 1942). Although considerable work has been done on the breaking-up of the sheath -in telophase with subsequent ag-gregation of the p a r t i c l e s to reform the nucleolus, { Nebel, 1939; Gates, 1942), the forming of the sheath by d i s i n t e g r -a t i o n of the nucleolus seems to have received less a t t e n t i o n except by e a r l i e r workers, ( Yari Gamp, 1924; Blackman, 1905), the l a t t e r m i s i n t e r p r e t t i n g the process to ind i c a t e the flow of chromatin to form the chromosomes. Z i r i l e , 1928, 1930; and Schrader, 1940 concluded that the nucleolus contains no eu-chre mat i n . P l a t e I I I , f i g s . 12 & 13, taken from diplotene, show the (supposed), t r a n s f e r of nuc l e o l a r m a t e r i a l to the chromo- J nemata. In the preparations, there could be detected no l i n e of demarkation between the green-staining m a t e r i a l of the nucleolus and that of the chromosome. In the f i g u r e s , the portions stained green are i n d i c a t e d by l i g h t areas and the euchromatin by dark areas. The non-staining, heterochromatic regions, (Schultz,, 1941), are obscured by the green-coloured sheath. According to t h i s author, the sheath may be s t r i p p e d o f f by enzymatic d i g e s t i o n using ribonuclease. Further dige s t -ion by the same enzyme r e s u l t s i n the removal of the nucleo-proteins leaving only the protamine core. 21. I l l , f i g s , 9-11 show pos s i b l e SAT chromosomes but no trabants were seen. 7. VI. Size Relationships of C e l l Parts As an i n d i c a t i o n of the a c t i v i t y of the various parts of the c e l l during the interphase and prophase stages, a number of measurements of c e l l s , n u c l e i and n u c l e o l i were made for the. period of February I I to March 17, 1944. The r e s t i l t s seem to be of- considerables i n t e r e s t as a means of approaching the problem of c e l l u l a r metabolism. Various workers, (Gates, 1942; "Schultz, 1941), have i n d i c a t e d that the interphase c e l l , , f a r from being at a " r e s t i n g stage', i s an extremely a c t i v e system. Schultz', 1941, points out that i n a c t i v e l y d i v i d i n g c e l l s the ribose n u c l e i c a e i d concentration i s r e l a t i v e l y , high. An examination of the measurements presented g r a p h i c a l l y i n t h i s paper, (Plates VI - I X ) , w i l l show that there are very appreciable changes i n s i z e of the c e l l constituents at t h i s time. Up to the beginning of the meiotic prophase - . there appears to be a d i s t i n c t reduction i n the amount of cytoplasm i n the c e l l . At the same time there i s a corres-ponding increase i n the nuclear, m a t e r i a l . This suggests that there i s a t r a n s f e r of. m a t e r i a l from cytoplasm to nucleus. I t was thought at one time that n u c l e i c acids migrate from the cytoplasm to the nucleus, but according to Gates, 1942, each has i t s own separate n u c l e i c a c i d c y c l e . However, according to Caspersson, (see Schultz, 1 9 4 1 ) . p r o t e i n synthesis proceeds hy way of the histones i n the nucleolus, which d i f f u s e through the nuclear membrane to form i n the cytoplasm rihose n u c l e i c acids which are concerned with the synthesis 8. of cytoplasmic p r o t e i n s " . This would i n d i c a t e that the nu c l e i c a c i d cycles are not independent. Schultz, 1941, presents- evidence that the "....gene....turns i t s synthesis ....to the increase of the n u c l e i c a c i d component....at the prophase of mitosis Cor meiosis}; or conversely during the interphase, the p r o t e i n -component i s synthesized." The data shown for the period, February 11-26, P l a t e 71, are taken to in d i c a t e the t r a n s f e r of materials from the cytoplasm to the nucleus preparatory to synthesis of n u c l e i c a c i d by the genes during prophase. I t w i l l be noted that there i s an apparent decrease i n the nucleolar volume at t h i s time which may. i n some way be concerned with the fu s i o n of the n u c l e o l i . How-ever t h i s may be accounted for i n the d i u r n a l v a r i a t i o n which i n some forms may be as much as BOfo^ ( see Gates, 1942). From leptotene to zygotene there i s a marked Increase i n the s i z e of a l l p a r t s , t h i s increase taking place at about the same rat e i n a l l three as in d i c a t e d by the slopes of the l i n e s . The sudden increase seems to be c o r r e l a t e d with the absorption of the i n t e r c e l l u l a r n u t r i t i v e m a t e r i a l and poss-i b l y with the r e c e i v i n g of contr i b u t i o n s of t a p e t a l m a t e r i a l . A f t e r zygotene there i s a sudden decrease i n . s i z e of a i l -pa r t s . This seems to be i n i t i a t e d at zygotene by the phenom-enon of s y n i z e s i s when the 'spireme" or chromatin network collapses to form the bouquet stage. I t seems l i k e l y that the r a p i d synthesis of n u c l e i c a c i d at t h i s stage i s accompanied by the l o s s of considerable metabolic waste? i n the form of water and di s s o l v e d s a l t s . This too could account for the 9. collapse of the spireme as the medium became le s s viscous and p o s s i b l e changes i n e l e c t r i c a l charge were effected.The change i s .greatest i n the nucleolus which may i n d i c a t e that t t h i s s tructure Is instrumental i n the synthesis of chromatin. From pachytene to diplotene there i s a s l i g h t increase i n a l l . p a r t s which may not he s i g n i f i c a n t . *A11 measurements were made i n groups of ten each t o t a l l i n g one hundred and f i f t y measurements. The graphs i n P l a t e s T i l - i s indicate' theedegree of r e l i a b i l i t y of the means ob-tained. The means are represented by h o r i z o n t a l l i n e s and c i r c l e s . The standard e r r o r s of the means are represented by the lengths of the v e r t i c a l rectangles. The ranges of meas-urements i n the samples are represented by the v e r t i c a l s i n g l e l i n e s . In P I . VII, i t i s clear that the means are consecutively d i f f e r e n t . e x c e p t f o r the l a s t two, CMareh 13-17). In other words, the rectangles do not overlap except i n the l a s t two. In the case of the nucleoplasm, P I . V I I I , the change i n the volume from February l l - 2 b cannot be considered s t a t i s t i c a l l y v a l i d because of the overlapping of the stand-ard error of the means. The inferences previously drawn from t h i s data must be considered t e n t a t i v e u n t i l f u rther obser-vations are made. The same i s true for the f i n a l two readings. The n u c l e o l a r changes shown i n P I . IX may a l l be considered v a l i d except the f i r s t change, (February 11-26). Error s of measurement, of estimation of volume, and v a r i a t i o n s due to other f a c t o r s such as d i u r n a l v a r i a t i o n have not been taken into account; i n these data. 10. V I I . Conclusion This paper has attempted, i n a preliminary way, to discuss the f o l l o w i n g aspects of microsporogenesis i n Abies grandis: 1. A "brief d e s c r i p t i o n of the stages of r e d u c t i o n - d i v i s i o n with drawings and photographs. 2. A study of the structure and development of'the nucleolus from the interphase where i t e x i s t s as seveaal d i s t i n c t n u c l e o l i , through pachytene where f u s i o n into a single nucleolus i s complete, to diplotene where i t d i s i n t e g r a t e s to form the sheath of the chromosomes. 3. A study of the t r a n s f e r of nucleolar m a t e r i a l from nucleolus to chromosome sheath with drawings as i l l u s t r a t i o n . 4. A s t a t i s t i c a l study of changes i n s i z e of the c e l l p a r t s i n the meiotic prophase and t h e i r s i g n i f i c a n c e i n the metabolism ox the c e l l . iv» BEFEEEN PES Blackman, M.W. 1905. On the karyosphere and nucleolus i n the spermatocytes of Scolopendra subsinipes. Proc. Am. Acad. Arts-& S o l . 41:329-544. Cowdry, E.V. and Kitchen, S.!1. 1930. Intranuclear i n c l u s i o n s i n yellow fever. Am. Jour. Hyg. 11:227-299. Gates, R.R. 1942. N u c l e o l i and r e l a t e d nucleolar s t r u c t u r e s , Bot. Rev. &( 6) :337-409... Guenther, E. 1903. Ueber den Nucleolen i n reifenden Echin-odermenei und seine Bedeutung. Zool. J a h r b . Abt. Anat. 19:1-28. Lucas, A.M. 1940. The cytology of fox e n c e p h a l i t i s end the e f f e c t s of cent i f a c t i o n on the intra n u c l e a r i n c l u s i o n s . Am. Jour. Path. 16:739-760 M c G i l l , G, 1906., The -behaviour of .the n u c l e o l i during oogenesis of the dragon-fly, w i t h s p e c i a l refernce to synapsis. Zool. £ahrb. Abt. Anat. 23:207-230. Nebel, B.R. 1939. Chromosome st r u c t u r e . Bot.Rev. 5_( 11) :563-626. P a i n t e r , T.8. 1943. C e l l growth and n u c l e i c a c i d i n the po l l e n . o f Rheo d i s c o l o r . Bot.Gaz. 105t1):56-58. Schrader, . IP. 1940. The formation of tetrads and the meiotic mitoses i n the male of Rhytidolomia s e n e l i s Say. Jour. Morph. 67:123-141. Schultz, J . 1941. Evidence of the nucleoprotein nature of the gene. Cold Spring Harbour Symposia on Quantitative Biology. V o l . IX. pp. 55-65, Semmens, C.S. and Bhaduri, P.N. 1941. Sta i n i n g the nucleolus. S t a i n Techno1. 16:119-120. Stedman, E. and Stedman, Mrs. E. 1943. Chromosomin, a p r o t e i n constituent of chromosomes. Nature 152:267. Van Camp, G.M. 1924. Le r o l e du nucleole dans l a caryocinese somatique ( O l i v i a m i n i a t a ) . La C e l l u l e 54:1-50... Z i r k l e , 0. 1928. Nucleolus i n root t i p meristem of Zea mays. Bot. Gaz...86:402-418. Z i r k l e , C. 1930. N u c l e o l i of the root t i p and cambium of Pinus strobus. Cytologia 2:85-105. V . Explanation of PI?tee Plate I. Interphase,(fig. 1 & .2), and prophase,(fig. 3 & 4) , stages, showing nucleolar structure. F i g . 1 & 2, February 11, 1944; f i g . 3 & 4, February 26, 1944. Magnification of drawings about 1500 X. Piste II. Fi£. 5, small portion of nucleus at zygotene to show pairing of, hpmologues, (March 4, 1944). F i g . 6, small portion of nucleus at pachytene showing begin-ning of r e l a t i o n a l c o i l i n g , (March 13, 1944). F i g . 7, diplotene nucleus with nucleolus 'discharging' i t s contents and nuclear membrane disappearing, (March . 17, 1944). F i g . 8, diplotene chromosome showing chromatic, (dark), and green-staining portions, (light Figs. 5-7 about 1500X; f i g . 8 about 6000X. Plate III. F i g . 9-11, possible SAT chromosomes around the nucleolus at diplotene. F i g . 12 & 13, transfer of nucleolar material to chromosomees, diplotene. A l l drawings magnified about 6000X. Plate IY. F i g . 14, f i r s t metaphase. F i g . 15, f i r s t telophase. F i g . 16, seoond anaphase. F i g . 17, second telophase. March 20, 1944. A l l drawings about 150QX. Plate V. Photographs of microsporocyte development showing nucleolar structure and numbers. F i g . 18, February 11, 1944. Fig. 19, February 26,1944. Fig. 20, March 4, 1944. F i g . 21, March 13, 1944. A l l photographs magnified about 800X. Plate VI. Changes i n volume during prophase of cytoplasm, nucleoplasm, and n u c l e o l i . Plate VII* Volumes of cytoplasm at different periods of prophase and significance of measurements. Plate VIII. Volumes of nucleoplasm at different periods of prophase and significance of measurements. Plate IX. Volumes of nu c l e o l i at different periods of prophase and significance of measurements. Plate I Plate II P l a t e I I I Plate IV P l a t e Y Plate VI Plate VII Plate VIII Plate IX 

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