254 bulletin: museum of comparative zoology. 



It was predicted from the position of the spindle in this cell that the 

 division would be unequal, the more centrally and dorsally situated 

 daughter cell being the smaller. This smaller cell is represented by c^"^^ 

 (Fig. 82), but only the upper end of its large sister cell, c^*^^, appears in 

 this section. In Figure 83 (Plate XL) we see the deeper portion of c^-^^, 

 which contains a nearly horizontal spindle. 



Posterior to the pair of cells just described are the descendants of C^-^ 

 (Fig. 71), the next to the hindmost of the mesencliyme cells in the left 

 half of the embryo. In Figure 7-4 they were in the eighth generation 

 (0^-^^ C*'-'^). One, the smaller, still remains in that generation (C^-^^ 

 Fig. 82), but its larger, more deeply situated sister cell has passed into 

 the ninth generation, and is now represented by C^-^", C®-^*, Figure 83. 

 The direct evidence of mitosis has not been observed for the division 

 here assumed, but very strong indirect evidence for it exists in the fact 

 that at the last cell division in the mesenchyme cells, C~'^ divided earlier 

 than c''-^^ (cf. Figs. 74 and 75). If the same order of division is followed 

 in case of the daughter cells, division ought to occur earlier in C^-^^ than 

 in c^-^^ But the latter cell is seen in Figure 83 to be in process of 

 division; therefore it is reasonable to suppose that at the same stage the 

 former cell has already divided. 



The small posterior mesenchyme cells, C-^, ZP-^, lie one behind the 

 the other deep down in the floor of the gastrula (Fig. 83), just posterior 

 to the endoderm cells and overlaid by muscle cells, — for such the in- 

 vaginated cells of the neuro-muscular ring become. 



These muscle cells have been crowded inward and downward at the 

 posterior margin of the blastopore in consequence of the rapid contrac- 

 tion of that opening. 



In the most posterior pair of muscle cells, viz. C-^, D''-^ (Fig. 71), 

 mitosis was observed to occur, as already stated, at a stage earlier than 

 this. The daughter cells arising from that division are readily recog- 

 nized in C8-", C«-i2, and i)^-", D^-^"" (Fig. 83). The nuclei of C^-" and 

 Z)^-" lie in the section intermediate between those represented in Fig- 

 ures 82 and 83. 



I am not able to declare with certainty the lineage of each of the 

 other muscle cells in this series of sections, so I shall not attempt to 

 point them out one by one. As a group, however, they are clearly 

 distinguished from the ectoderm cells on the one hand, and from the 

 mesenchyme cells on the other, by their large nuclei, their considerable 

 size, and the peculiar stainability of their protoplasm. They resemble 

 very closely in stainability the nerve cells lying anterior to the blasto- 



