90 CHIM^ROID PISHES AND THEIR DEVELOPMENT. 



entoderm prevail; the thickened ectoderm at ect marks a point at which this layer 

 is making cellular additions to the mesoblast; it represents the marginal point 

 where the tail fold and the margin of the blastoderm meet. At other points also, 

 the mesoblast is receiving increments ; in addition to the gastral mesoblast we note 

 cells arising from the wall of the yolk-entoderm midway between the cavity of the 

 gut and the periphery of the blastoderm, and we see further that an invasion of 



Figs. 72 F-K. Details of the region of the yolk-entoderm of fig. 72. In /' the region is indicated in detail which lies immediately 

 below and at the side of the arching wall of the gut. (Cf. fig. 72 E.) 



" Large vacuolar nucleus which appears on the point of undergoing reconstitution Wl. Megasphere. 



into a cell of the yolk-entoderm. SffZ. Subgerminal zone, 



ft. Nucleus similar to foregoing, but in a less advanced condition. V. Vacuolar zone. 



r, c', (1. Cells which have recently been differentiated out of the germinal wall. ?/''- Yolk entoderm. 



G, H, and I illustrate particularly the zone of reconstruction of yolk-entoblast cells from yolk nuclei. In J a telophase occurs, repre- 

 senting a rare condition in the subgerminal zone. In K, similarly, a telophase occurs in a megasphere. The latter has, however, 

 passed through the zone of vacuoles and lies in the yolk-entoderm. In this neighborhood, however, as we note at the left, a 

 syncytial condition may be present. 



cells from the periphery of the blastoderm has occurred, in the form of a crease- 

 shaped invagination. In c the dorsal wall is sharply distinguished from the sides of 

 the gut. On the floor of the latter appear small masses of yolk, y, which can only 

 serve, as already noted, as ingested nutriment. The mesoblast in this region 

 shows considerable differentiation; myotomes are sharply marked off; the gono- 



