DETAILS OF EARLY EMBRYO. 91 



nephrotomal zone is of notable size; at d and in the adjacent cell-mass (at the left) 

 are the beginnings of the pronephric tu])ules; and below at x appears the thickcnmg 

 of the mesentoderm whence arises the posterior portion of the pronephric duct. 



In a section, d, passing through a more anterior region of the embryo, the 

 urogenital structures are practically undifferentiated; the mesoblast extending 

 continuously from the notochord to the periphery of the blastoderm. In this 

 region the mesoblast probably receives little or no increment from the yolk-ento- 

 derm, judging from the latter's smooth surface, save only at or near the margin of 

 the blastoderm. Below the yolk-entoderm in this region the subgerminal zone of 

 nuclei is more conspicuous and definite than in the early stage, fig. 7 1 e, and this 

 zone, indeed, appears with even greater prominence in the more anterior section, 

 fig. 72 E (to be contrasted with fig. 71 F or g). It will here also be seen that divi- 

 sion of the mesoblast into splanchno- and somatopleure is occurring, and that the 

 lateral wall of the gut is more definitely established. 



A detail, shown in f, indicates the more special relation of the subgerminal 

 zone to the marginal cells of the gut cavity. The subgerminal zone is here reduced 

 to a narrow tongue {cf. also e), which inserts itself under the thickened mass of cells 

 at the base of the gut wall, in the direction of the lumen of the gut. In the present 

 detail the base of the gut wall is shown at gw, the yolk-entoderm at ye, the 

 vacuolar layer at v, and the subgerminal zone at sgz. We note first of all the narrow- 

 ness of the vacuolar layer, through the intervention of which we have seen (fig. 71 o) 

 yolk nuclei become cells of the embryo, a condition indicating the specialization of 

 this region. In this zone {v), furthermore, we see large nuclei which are evidently 

 in transition between yolk and embryo, and at m a megasphere which has just 

 passed through it, the vacuoles becoming reconstituted below. Most significant in 

 the region of the rim of the gut wall is the concentration of the elements of the 

 subgerminal zone, coarse yolk, fine yolk, lacunae, vacuoles and yolk nuclei of different 

 kinds, the continuation (to the left) of the vacuolar layer, and the compounding of 

 its vacuoles — characters which are obviously to be interpreted as more special and 

 complicated than in the earlier stage. 



A few additional details may be cited. In g, where nuclei are passing through 

 the vacuolar zone and becoming cells, we observe that at c a nucleus which has 

 been taken into a large vacuole (a process forming now a reconstituted cell), is still 

 dividing amitotically, and that at d a similar division has recently occurred, indi- 

 cating in both cases, as we have before remarked, that the difference between ami- 

 totic and mitotic division is one of degree rather than of kind. In 11, a detail from 

 a section close to fig. 72 E, a point is figured where merocytes and newly constituted 

 yolk-entoderm cells occur in such confusion that it is difficult to say where the layer 

 of merocytes terminates and where the cells of the embryo begin. And the same 

 is true of the detail shown in i. In the last figure, on the other hand, merocytes 

 are still multiplying, even at a point close to the yolk-entoderm. In j, a detail of 

 the vacuolar region, cells are arising from merocytes; at /^ a merocyte, less vesicular 

 than a, adjoins a vacuole into which it will probably pass, judging from transitional 

 conditions {cf. the neighboring c). And even in the vacuolar layer such newly 



