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CHAPTER VI. 

 EARLY DEVELOPMENT OF THE CHICK. 



Probably every student has seen a hen's egg, or the egg of some other 

 bird, and knows its size, shape and appearance. If the calcareous shell is 

 broken, the whitish shell-membrane is found closely applied to the inner 

 surface. Enclosed by the membrane is the glairy, transparent '" white" 

 or albumen. Through this can be seen the yellow spherical yolk mass. It 

 requires close observation to discern the delicate transparent vitelline mem- 

 brane around the yolk. If the egg has been in one position for a few minutes 

 the tiny white germ disk, less than a quarter of an inch in diameter, will 

 appear on the upper side of the yolk. The yolk mass and germ disk to- 

 gether constitute the ovum which was discharged from the Graafian follicle of 

 the ovary. The vitelline membrane is a true cell-membrane, a product of the 

 egg cytoplasm. All the structures on the outside of this are secondary egg- 

 membranes deposited by the epithelium of the oviduct as the ovum passed 

 along. If the egg has been fertilized before it is laid the germ disk represents 

 a considerably advanced stage of development, for fertilization occurs in 

 the extreme upper end of the oviduct and during the time the egg is traver- 

 sing the tube, a period of about 24 hours, the early formative processes have 

 gone on. In order to observe the earliest stages it is necessary therefore to 

 obtain and study the egg before it is laid. 



In the chapter on the germ cells it was pointed out that the bird's egg 

 represents the polylecithal type in which the quantity of yolk or deutoplasm 

 reaches the maximum (p. 6) . The cytoplasm, with the nucleus, comprises 

 a small disk, about 3 mm. in diameter and 0.5 mm. thick, which rests upon 

 the yolk. The bulk of the yolk contains no cytoplasm at all, and the transi- 

 tion from pure yolk to pure cytoplasm is rather abrupt. The vast accumu- 

 lation of yolk in the bird's egg is correlated with the long period that the 

 growing embryo remains within the shell when it must depend upon an in- 

 ternal food supply. The reptilian egg represents the same type. The 

 course of development is greatly modified by the yolk content, and since 

 the mammals are probably descended from forms (reptiles) whose eggs 

 contained much yolk, a study of the developmental processes of a polyleci- 

 thal egg throws much light upon the development of mammals whose eggs 

 contain but little yolk although resembling in their mode of development 

 their ancestral type. 



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