74 



CHIM^ROID FISHES AND THEIR DEVELOPMENT. 



To make the comparison of the gastrulae of Chima^ra and Shark more concrete 

 we have figured two stages side by side (fig. 67, a and c, b and d). We need 

 only add to the foregoing text the remark that the archenteron and segmentation 

 cavity are more distinct in Chim?era, and that the differentiation of the embryo 

 takes place in a more restricted area. We append also (fig. 68, a, b, c) a scheme 

 expres.sing our interpretation of the mode of origin of the meroblastic gastrula in 

 this form. In a is pictured a sagittal section of an early gastrula of a holoblastic 

 type, and between the points marked with asterisks is indicated the narrow zone 

 below which the amount of 3'olk is supposed to have notably increased. In b, the 

 second stage in this evolution, is a condition not unlike the late gastrula in Chimai-ra: 

 The yolk mass still segments, and the ventral lip, vl, passes inward and forward as 

 the dorsal lip rolls backward and inward. In c, finally, is attained the condition in 

 sharks: Archenteron and segmentation cavity merge; segmentation is lost in the 

 $c 



arch 



Fig. 68. — Diagrams suggesring origin of meroblastic character of egg of sfiark. 

 A. Sagittal section of early gastrula of holoblastic egg (e. g., Petromyzon). _/>. Section showing conditions similar 

 to those in Chimaera colliei (cf. fig. 66). i\ Section of gastrula of shark, afch, Archenteron; r'/, ventral 

 lip of blastopore ; .s-r, segmentation cavity. 



j'olk mass, and the latter comes to pass its nutriment into the blastoderm indirectly, 

 i. c, as nourishment for the growth and multiplication of the cells already formed, 

 instead of directly, ?'. e., in the form of new yolk-filled blastomeres, and from this 

 process there results a smooth germinal wall. This interpretation agrees in general 

 with that lately restated bj" Ziegler (Lehrbuch Entwicklungsgeschichte, 1901, pp. 

 352-353); it difTers in the interpretation of the fate of the ventral lip of the 

 blastopore. According to the older view the ventral lip remains more or less 

 passive, in the present interpretation it has undergone a marked change; the cells 

 which primitively formed the ventral lip of the blastopore are to be sought in the 

 region vl, on the floor of the archenteron. The dc facto ventral lip of the 

 blastopore (/. e., in all stages but the earliest) is accordingly a secondary structure, 

 which arises from the new conditions attending the overgrowth of the blastoderm. 



