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Abstract

A study was carried out to determine if genetic variation existed in the rate of embryonic development of a line of chick embryos of the New Hampshire breed, and to examine the extent to which variation in early developmental rate, including that attributable to genotype, could be related to post-hatching growth of sibling chicks. Indices of developmental rate examined in individual embryos were a) .cellular proliferation rate between 77 and 98 hours of incubation of specific tissues within the embryonic central nervous system and of embryonic blood cells, and b) total protein content, embryo dry weight, Hamilton-Hamburger stage, total lactic acid dehydrogenase (LDH) activity and expressions thereof, and total alkaline phosphatase (AP) activity and expressions thereof, at 96 and 112 hours of incubation. Sire differences existed in the absolute rate of cellular proliferation, as determined by the technique of autoradiography, of the neural tube and metencephalon and, possibly, the myelencephalon. No such differences in the activity of the mesencephalon, diencephalon or embryonic blood cells were detected. A large maternal influence on the proliferative activity of all tissues examined was felt to exist. Sire differences in the protein content of 112-hour embryos were observed and may also have existed with respect to 96-hour protein content and embryo dry weight but were masked by maternal factors. The heritability of 96-hour protein content was estimated to be .412; the 112-hour estimate was .198. Corresponding values for 96-hour and 112-hour total LDH activity were .294 and .344, respectively. Standard errors of all heritability estimates were very high. A sire effect on 96-hour total LDH activity was indicated and there was evidence to support the existence of a similar effect on 112-hour Hamilton-Hamburger stage; maternal factors did not appear to have influenced either of these embryonic traits at either incubation stage examined. Sire differences in total AP activity per unit of supernatant protein (AP/SP) determined at 96 hours of incubation were observed. The heritability of 96-hour total AP Activity was .554 and the corresponding 112-hour value was .114. Sire differences existed for both the absolute level of LDH₁ activity and LDH₁ per unit of total protein. Both the proportionate and absolute amount of LDH₁ activity decreased between 96 and 112 hours of incubation. The activity of this isozyme was concluded to be directly associated with proliferative activity at both incubation stages examined. It was positively related to protein content and total LDH activity and was inversely related to both LDH concentration and LDH₅ activity. Total LDH activity and protein content were highly correlated in a positive fashion at both incubation stages. Protein content was also positively correlated to Hamilton-Hamburger stage at both stages while total LDH activity and Hamilton-Hamburger stage were correlated at 96 hours of incubation only. A negative linear association was found between post-hatching growth rate of males and the proliferative activity of all embryonic central nervous system tissues examined (except the diencephalon), and between female body weight to four weeks of age and the activity of embryonic blood cells. There was also a negative, though not always linear, relationship between post