282 BASHFORD DKAN. 



ily identified with that in the earlier stage. In an adjoining sec- 

 tion, of which a detail is given in Fig. I I A, a rudiment of the 

 blastopore of the earlier stage is shown in the region BP. 



In a still later gastrula, Fig. 12, the prominent feature is 

 the growth of the blastoderm hindward. The blastoderm has 

 now increased its diameter by about one third, and its surface 

 growth has outstripped that of the subgerminal region in spite 

 of the fact that the subgerminal wall has notably flattened. Thus 

 it has come about that the posterior region of the blastoderm of 

 the earlier stage has rolled over, as it were, the neighboring ger- 

 minal wall. The point indicated at BP. has thus come to lie first 

 at the edge of the blastoderm, and is next passed under its rim. 

 And as at this point of " invagination " a separation of the cells 

 now occurs, this may be taken as the reopening of the blastopore. 

 With this rapid hindward growth the mass of cells, PM., is seen 

 to take a position apparently far forward on the subgerminal wall, 

 but it remains in reality in its previous relative position (i. e., it 

 retains the same actual distance from the anterior border of the 

 blastoderm). Connected again with this backward growth of 

 the surface of the blastoderm it is also evident that the general 

 lie of the archenteric cavity has changed, its anterior end having 

 now been carried backward, its main length (axis of cavity) rotat- 

 ing somewhat as denoted by the arrows in the figures. Other 

 symptoms of the rapid growth of the posterior rim of the blasto- 

 derm are seen in its closely compressed cells, epithelial, and in its 

 crumpled condition. Noteworthy, further, is the solidness of the 

 mass of cells roofing the anterior portion of the archenteron, and 

 the reduced size and clearer contour of the segmentation cavity. 

 It will be observed that in this stage the conditions are closely 

 similar to those of the shark, excepting only that the archenteric 

 and segmentation cavities have not clearly merged. 



Early Embryos. Early embryos resemble closely those of 

 typical elasmobranchs (cf. Figs. 3, 4, 15); the entire extent of 

 of their surrounding blastoderm, however, is much smaller, and 

 the embryo attains a large size before the adjacent blastoderm 

 surrounds a small mass of the yolk (about three eighths inch in 

 diameter). Stages occur resembling closely sharks in Balfour's 

 notation C, D and E. One might note that the head folds in the 



