XXIV 



DEVELOPMENT OF THE CHICK, 

 CONTINUATION OF THE GENETICS 

 EXPERIMENT 



(Readings: J. D. Ebert, "The First Heartbeats," Sci. Am. 200, No. 3, 87-96, 

 March 1959, Reprint No. 56. C. H. Waddington, "How Do Cells Differentiate?" 

 Sci. Am. 189, No. 3, 108-116, Sept. 1953, Reprint No. 45. See also the handsome 

 photographs showing the progressive stages of chick development in the little 

 book by E. Bosiger and J. M. Guilcher, A Bird Is Born, Sterling Pub. Co., 1959.) 



1... 



The chick egg has been a classic object for 

 the study of embryonic development for the last 

 three-hundred years. It achieved this position 

 in the great work of Wilham Harvey (whom 

 you already know as the discoverer of the cir- 

 culation of the blood) on The Generation of 

 Animals. This work contains on the title page 

 the aphorism, Ex ova omnia, "all life from the 

 egg." The chick egg provides fine material for 

 the analysis of development beyond the earliest 

 stages, which have already passed before the 

 egg is laid. 



The egg is fertilized immediately after ovula- 

 tion, as soon as it enters the oviduct. Usually 

 five or six sperm enter, a common condition in 

 the large eggs of certain amphibia, reptiles, and 

 birds, though abnormal in most other animals. 

 One sperm head eventually fuses with the egg 

 nucleus; the others disintegrate. 



At the time of sperm entry the egg nucleus is 

 just entering its first maturation division, and 

 must go on to complete its meiosis before the 



egg and sperm pronuclei fuse. Then cleavage 

 begins, and goes through to early gastrulation 

 within the hen, before the egg is laid. Also the 

 walls of the oviduct secrete a layer of albumen 

 around the egg, which serves later to float the 

 embryo within the shell and provides it with an 

 aqueous environment. The shell membranes 

 and the porous limestone (calcium carbonate) 

 shell are subsequently laid down by the shell 

 gland. Only the yolk with its small disc of 

 protoplasm represents the true ovarian egg. All 

 the rest is accessory structure. All vertebrate 

 embryos develop in an aqueous environment; 

 and such eggs as this represent a device for 

 bringing and maintaining an aqueous environ- 

 ment ashore — in a sense, an enclosed pond. 

 How do you think the size of the yolk is corre- 

 lated with the time it takes various types of egg 

 to develop? 



In today's experiment each student can 

 examine an early stage in the development of 

 the chick embryo. Record your observations 



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