. Chimæroid fishes and their development. Fishes; Chimaeridae. 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 our interpretation of the mode of origin of the meroblastic gastrula in this form. In
. Chimæroid fishes and their development. Fishes; Chimaeridae. 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 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 inte
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