DEVELOPMENT OF A CHIM/EROID. 28 1 



face at the pore, BP. Behind it, /. e., between it and the yolk, 

 lie several rows of cells. In this cavity and its surface open- 

 ing, then, we recognize archenteron and blastopore, and note 

 further that the peculiar pigmentation of the cells of the surface 

 of the gastrula can be followed down on either side, suggesting 

 recent invagination, into the archenteric cavity, half way to its 

 bottom. This, therefore, we conclude, is not a mere intercellular 

 space with a fortuitous opening to the surface, but a definite 

 cavity, whose cells lining the outer half are pigmented. Its 

 cellular wall, moreover, is in general firm and compact, epithelial 

 in character. Accordingly we must admit that in Chimera 

 a gastrula is formed whose blastopore is located not at the 

 rim of the early blastoderm but near it. And it follows that 

 in this condition there is still retained a phylogenetic stage in 

 elasmobranchian gastrulation wherein the merging of the blas- 

 toderm cells into the surrounding yolk has not yet extended 

 to that zone of the blastoderm where the blastopore is form- 

 ing. It will be observed that growth is taking place at both 

 anterior and posterior margins of the present blastoderm. Cells 

 are coming to be budded off behind the archenteron, as well as 

 at the blastoderm's anterior rim (Fig. 10 B) within the overlying 

 germinal wall itself (v. between the points * and *). And even 

 at this late stage blastomeres are found to be budded off abun- 



o 



dantly from the subgerminal wall. One notes, further, that the 

 cavity at SC. enlarges considerably on either side of the median 

 line and is obviously interpreted as the segmentation cavity. In 

 this stage many merocytes and sperm nuclei occur in the germi- 

 nal wall. 



In Fig. f i is shown a somewhat later gastrula. Here the 

 compact character of the former stage is lost, the diameter of the 

 blastoderm having doubled. In the anterior region the segmen- 

 tation cavity, SC., is enlarged and is many-branched. The blas- 

 topore is obliterated, owing probably to the backward growth of 

 the cells at the surface of the blastoderm ; the archenteron, on 

 the other hand, was greatly increased in size ; its anterior wall, 

 spreading out into a thin, somewhat epithelial layer, forms its roof, 

 and its fundus is made up of loose cells, which focus together 

 anteriorly. Behind the archenteron is a mass of cells, PM,, read- 



