124 



Finally, in the early development of the mesoderm amitoses are 

 more abundant than mitoses (sers. 328, 189). However, the percentage 

 of mitoses is relatively high, a condition due to a local variation in the 

 two types. Thus, in the central part of the primitive streak where 

 the mesoderm is taking its rise, and consequently cell multiplication 

 is very rapid, nuclear division is almost wholly amitotic, while in the 

 lateral parts of the wings of the mesoderm mitosis predominates. 

 This condition indicates that cells which have undergone amitosis are 

 later dividing mitotically, because the cells composing the wings of the 

 mesoderm are the progeny of those previously situated in the central 

 part of the primitive streak. 



However, those who still cling to be theory of Ziegler and vom 

 Rath may argue that the few cells in indirect nuclear division in ra- 

 pidly growing regions are the progenitors of the more numerous mito- 

 tically dividing cells, which are present when these same regions have 

 later entered a period of slower growth. Against this possibility two 

 objections may be urged. In the first place, according to the idea of 

 Ziegler and vom Rath, the progeny of a cell which has undergone 

 direct division must always thus divide. We ought to expect then a 

 progressive increase in amitotically dividing cells, but instead we find 

 a decrease when a region passes from a period of rapid to one of 

 slower growth. Apparently the most plausible explanation that might 

 be offered to account for this decrease, would be to suppose that some 

 of the amitotically dividing cells, after undergoing direct nuclear di- 

 vision for a time, cease to divide at all, that is, they have become 

 either too degenerate or too highly specialized. There is no 

 evidence that either of these alternatives is true, for no trace of de- 

 generative cells has been observed, and one would hardly expect to 

 find in these early stages differentiation so far advanced as to have 

 produced special cells no longer capable of undergoing even so simple 

 a process as that of direct nuclear division. 



In the second place, 1 have discovered cells in the mesoderm, 

 which seem to indicate very clearly that mitosis occurs in a cell after 

 it has divided amitotically. In these cells one of the daughter nuclei, 

 which have been produced amitotically, is preparing to divide by mi- 

 tosis before the cytoplasmic division is completed. A series of such 

 cells is shown in Figs. 20 — 22. In Fig. 20 the daughter nuclei are 

 the products of amitosis, and the lower nucleus is apparently in an 

 early prophase stage (cf. F'ig. 24). Judging from the appearance of 

 the lower nucleus in Fig. 19, this cell may be interpreted to be in a 

 stage less advanced than that of Fig. 20. Although it is more difficult 



