248 OUTLINES OF CHORDATE DEVELOPMENT 



while its circular margin, apparently derived largely from the 

 periblast, is called the area opaca. The ring-like periblast 

 continues to grow and to become nucleated more peripherally, 

 at the same time that it is contributing cells to the blastoderm, 

 so that it steadily increases in diameter. The inner nucleated 

 margin of the periblast, which becomes of cellular composition 

 and contributes to the later extra-embryonic tissues, is known 

 as the germ wall (Fig. 91). Later the cells of the blastoderm 

 extend peripherally, overlapping the inner margin of the germ 

 wall, so that there is a narrow region transitional between pellu- 

 cid and opaque areas. We should not overlook the fact that 

 the lower surface of the periblast is directly continuous with the 

 yolk-mass, and is peripherally continuous with a very thin 

 superficial layer of protoplasm; this latter is sometimes re- 

 ferred to as a part of the germ wall. The thinning of the blasto- 

 derm, mentioned above, may be regarded as indicating the 

 completion of the blastula stage and the beginning of gastrula- 

 tion, to which we may next turn our attention. 



D. GASTRULATION 



In the chick the formation of the primary germ layers, ecto- 

 derm and endoderm, i.e., gastrulation proper, is quite easily 

 distinguished from the processes of notogenesis and mesoderm 

 formation. In the following account of gastrulation many 

 details are supplied from Patterson's account of this process in 

 the pigeon, for this period in the development of the chick is 

 incompletely known, although it is clear that the pigeon and 

 chick are in agreement as to essentials. The thinning of the 

 blastoderm through the rearrangement of its cells, continues 

 rapidly as the diameter of the area pellucida increases. It is 

 most marked in a wedge-shaped area in the posterior part of the 

 blastoderm, the most posterior portion of which may come to be 

 only one cell in thickness (Fig. 92, A). In this region the sub- 

 germinal or segmentation cavity increases in depth, and around 

 the hinder margin of the thinner area the germ wall (area opaca) 

 appears to be interrupted more or less completely, so that the 

 blastoderm cells border directly upon the yolk. 



