CHIM^ROID FISHES AND THEIR DEVELOPMENT. 



at the point e a mitosis is taking place preliminary to budding off another mesoblast 

 cell. We conclude that the cell e' has been derived from the layer e, and not from 

 the cell mass m, when we consider (i) that its granular contents agree in character 

 with the layer e rather than with the mass m ; (2) that a continuous boundary line sepa- 

 rates the mass m from /; and finally (3) that the cell e' is connected with the layer 

 by a protoplasmic process, above which a nucleus in mitosis is present. Less 

 evident, from this section at least, is the question whether cells are added to the 



AA 



CC 



HH 



II 



BB 



DD 



E 



FF 



GG 



o. 



Figs. 71 O-II. Details in sections of foregoing embryo (continued from page 83). 



Detail showing transition between yolk region and the cells of the blastoderm. 



. Yolk nucleus lying against the wall of the vacuole v'. V. Zone of large vacuoles. 



Wl'. Yolk nucleus Dow lying within a vacuole, and transformed into .'/. Zone of small vacuoles and fine germinal yolk. 



a yolk-surrounded blastomere. 2/C- Yolk entoderm. 



P. Detail of blastoderm, showing at y the division by mitosis of a megasphere lying in the yolk entoderm. 

 Q. Detail of section near the marginal region of the blastoderm, showing single megasphere, y, lying free in the space between 

 ectoderm and yolk entoderm. 



yn. Yolk nuclei undergoing division by atypical mitosis and by amitosis. 

 R-Il. Details showing various phases of division in yolk nuclei. 



peristomial mesoblast from the entoderm more proximal in position. At some 

 points one is inclined to admit that such a cell as shown in j, ent, is being budded off 

 into the tongue of mesoblast. (Cf. the condition shown in the section N.) 



In the section F, the notochord is again continuous with the entoderm; the 

 gut region rises, and its lumen is now walled with cells save in its median-ventral 

 line. Here a thin wedge of yolk intrudes. Especially noteworthy is the relation 

 of the yolk to the yolk-entoderm in this region. The latter has again a more 



