GASTRULATION. 



63 



and coarse yolk is composed half of fine and half of coarse yolk. It is quite prob- 

 able, therefore, even from this single observation {cf. also infra), that the region of 

 the coarse yolk is not as inert as one is at first inclined to believe, an induction 

 which suggests at once that the fewer and larger fissure-like vacuoles in this region 

 are equivalent to the vacuoles of the fine yolk, or in other words, to intercellular 

 spaces. 



A final point of contrast between the foregoing stages: In the former the 

 blastomeres are relatively compact; in the latter there is a general inter-blastomeral 

 space which marks an early state of the definite cleavage cavity. It is probable, 

 as noted for the former stage, that the anterior end of the germ can now be 



distinguished. 



GASTRULATION. 



The stage shown in surface view in plate v, fig. 30, and in sagittal section in 

 fig. 63, is probably the most valuable of the author's early Chimasroid embryos. 

 For it may be accepted as providing a key to the problem of gastrulation not only 

 in this form but in sharks as well. Its discovery is none the less a fortunate one, 

 since it is a stage which has every appearance of being brief, and therefore easily 

 overlooked. In diameter it differs little from the blastula above described (fig. 62), 

 but its depth is notably greater. Comparing these two stages, we conclude that 

 the deep subgerminal region of the earlier stage (fig. 62), which was traversed 

 by vacuoles, has been replaced by the deep-lying mass of cells of fig. 63. We 

 observe that this thickening of the cellular mass has not yet been accompanied by 

 an extension over the surrounding region; the mass is at present compact, sub- 

 spherical, lying in a smooth depression of the germinal wall. At one end of the 

 cellular mass the segmentation cavity, below the letters sc, represents all that 

 remains of the intercellular spaces of earlier stages. Near the opposite end is a 

 small archenteric cavity, a, communicating with the surface through the opening 

 bp. The archenteron is regular in outline, its marginal cells forming a somewhat 

 epithelial lining (fig. 63 b). It has probably arisen by an invagination in pre- 

 existing cells, since the cells lining its outer half are slightly pigmented and closely 

 resemble those of the surface of the blastoderm. Especially noteworthy is this — 

 that behind the archenteron, /. c, between it and the germinal wall, are several 

 rows of cells. 



We have, therefore, evidence that in Chima^ra a gastrula is formed whose 

 blastopore is located not at the rim of the early blastoderm but near it. // 

 is thus a condition in zvJiich the merging of the cells of the blastoderm ivitJi the 

 surrounding yolk does not yet take place in that zone of the blastoderm in tchich the 

 archenteron is forming. We have here, accordingly, a condition which throws 

 light upon the origin of the gastrula of sharks, confirming in a striking way the 

 interpretations of C. K. Hofl'man (1896, Morph. JB., p. 210). 



