DEVELOPMENT OF OTHER VERTEBRATES 427 



tion, the ovum breaks from the surface of the ovary, as does the 

 ovum of the frog, and is, therefore, free in the coelome, although 

 the ovary is so closely enfolded by the funnel-shaped mouth of 

 the oviduct that the o\ami has httle chance of not reaching its 

 proper destination. The two maturation divisions (c/. Fig. 114, p. 

 228) occur after the egg has reached the o\'iduct and do not begin 

 until the entrance of a spermatozoon. The fertihzed egg, or 

 zygote, therefore, consists of the germinal disc resting upon a 

 spherical mass of nutrient material which is the "yolk" of the 

 future "egg." This "yolk" now passes along the oviduct, which 

 secretes first the "white" or albumen with the buffer-Hke chalazce 

 that are attached at opposite sides of the yolk, then the shell mem- 

 branes, and finally the shell. In this manner the hen's egg with its 

 familiar structure is produced. One may, therefore, compare the 

 "yolk" with a greatly enlarged ovum, and its several coverings 

 with the jell}^ of the frog's egg. 



The cleavage and initial development of the hen's egg consist 

 of cell divisions and differentiation in the blastodisc (Fig. 224 C), 

 beginning while the egg is in the upper part of the oviduct and con- 

 tinuing until it is laid, when the development is checked by the 

 lower temperature of the outer world. As these changes progress, 

 a shallow cavity, comparable with the blastoccele of a frog's egg, 

 is formed beneath the blastodisc, or blastoderm, as it may be 

 called from this time onward. The blastoderm then becomes a 

 two-layered structure, by a process that may be called gastrula- 

 tion, since the outer cells can be subsequently identified as the 

 ectoderm and the inner ones as the endoderm of the developing 

 embiyo. WTien this initial development ceases the embryo con- 

 sists of a mass of cells which is thinner and more transparent in 

 its central portion, called the area pellucida, and thicker around 

 the margin in the area opaca, as may be seen in the fertile egg when 

 viewed with the unaided eye (Fig. 224 B). Unfertilized eggs 

 have a spot at the animal pole that is quite different in appear- 

 ance. In this stage the development ceases unless the egg is kept 

 at a suitable temperature in an incubator or under a hen. If such 

 an egg is placed under incubation, the blastoderm develops rapidly, 

 extending over the surface of the yolk and producing an embryo 

 in the center of the disc-hke blastoderm in the manner shown in 

 Fig. 224 D to F. 



The more important internal changes by which the body of the 



