AVES 



317 



the shell membrane is a second layer of albumen which is quite fluid 

 in consistency. Surrounding the albumen is the double parchment- 

 like shell-membrane, with an air-space between its two layers at the 

 broad end of the egg. The shell proper is a rather complex structure 

 composed of calcium carbonate; it is porous and more or less pig- 

 ment ed. 



Cleavage (Fig. 161) is strictly meroblastic, the first cleavage being 

 merely a furrow, and many furrows are formed before any of the 

 cells are furnished with floors or bottom partitions that cut them off 

 from the underlying yolk. Development proceeds beyond the gas- 



B 



FIG. 162. Hen's egg at about the twenty-sixth hour of incubation, to show 

 the zones of the blastoderm and the orientation of the embryo with reference to 

 the axis of the shell. B, yolk of hen's egg incubated about 50 hours to show the 

 extent of overgrowth of the blastoderm. A. C, Air chamber; a. p, area pellucida: 

 a. v, area vasculosa; a. v. e, area vitellina externa; a. v. i, area vitellina interna; 

 Y, uncovered portion of the yolk. (From Lillie, after Duval.) 



trula stage before the egg is laid. A newly laid egg shows the embryo 

 as an embryonic disc, a small whitish spot at the animal pole, com- 

 posed of central transparent area (area pellucida) bounded by an 

 opaque ring or germ wall. The pellucid area is two-layered poste- 

 riorly, an inrolling of cells having occurred which constitutes the 

 primitive endoderm and is the equivalent of the archenteron in- 

 vagination of the frog. The blastopore is crescentic, as in the frog, 

 and the primitive streak is formed by concrescence of the blastopore. 

 The head forms in front of the primitive streak, which constitutes the 

 axis of the trunk. 



