EARLY DEVELOPMENT OF THE CHICK. 



69 



the Amphioxus' ovum there is only a small quantity of yolk, but enough to 

 result in the formation of a few larger cells in the blastula which are homo- 

 logous with the macromeres in the frog and the yolk mass in the bird. The 

 blastoccel in the bird is reduced to a minimum owing to the fact that the 

 cytoplasm comprises only a small disk which rests upon the relatively great 

 mass of yolk. (Compare Figs. 20, 29 and 39.) 



Before going on to the process by which ectoderm and entoderm arise, 

 it is necessary to consider briefly the transition between the margin of the 

 germ disk and the yolk. The incompletely divided marginal cells, which are 

 larger than the central cells, border upon the unsegmented yolk around the 

 disk. This yolk at first contains no nuclei and is called the periblast ring. 

 Then as the marginal cells continue to divide, the peripheral daughter nuclei 

 migrate into the periblast which thus becomes a nucleated yolk ring but is 

 yet wholly unsegmented and merged with the yolk. This nucleated yolk 

 ring receives the name germ wall. The nuclei here continue to divide and 



FIG. 40. Cross section through the center of the blastoderm of a pigeon, 14^ hours after fer- 

 tilization. Blount, from Lillie. i, Marginal cells; 2, periblast; 3, nuclei in the subgerminal 

 region. 



then cell boundaries appear. Some of the boundaries become complete, 

 and discrete cells are thus formed which apparently join the group already 

 forming the, cellular disk or blastoderm. Other daughter nuclei migrate 

 farther and other cells are formed and added to the margin of the blasto- 

 derm (Fig. 40). In this manner the blastoderm increases in size, extending 

 in all directions farther over the yolk. It is probable that the cytoplasm 

 of these cells is capable of using the yolk to build up into more cytoplasm, 

 a process comparable With digestion and assimilation. 



It is possible at this time, or even before, to determine the position of 

 the future embryo relative to the disk. If the disk is viewed from above in 

 its natural position in the shell, with the larger end of the egg toward the 

 left, the edge of the disk toward the observer will indicate the caudal end of 

 the embryo, and the long axis of the embryo will lie at right angles to the long 

 axis of the egg. 



