156 A. RICHARDS. 



might arise de novo, by migration, by amitosis and by mitosis. 

 The first of these possibilities was dismissed as highly improbable; 

 the burden of proof is upon him who upholds this view. The 

 second method of origin was admitted to be possible, but it was 

 thought to play only an unimportant role, if any at all, in the 

 development; and even if the assumption of migration as a 

 factor positive proof would scarcely be procurable were found 

 to be necessary it would merely throw the question of division 

 back into the earlier stages. It was held, therefore, that the 

 cells under discussion must arise in situ by direct or indirect 

 division. It is the purpose of this summary to present the evi- 

 dence for the two views as concisely as may be. 



It has been shown, furthermore, that at two periods of de- 

 velopment only may active growth by cell division be looked for: 

 namely, during the period of pre-oogonial and oogonial divisions 

 and during the cleavage of the embryo; the rest of the growth 

 involved is that of increase in size and in differentiation. The 

 discussion, therefore, limits itself to the question: is amitosis or 

 mitosis the method by which the pre-oogonial and oogonial and 

 the cleavage divisions occur? 



The evidence, as I have found it, favoring the view that ami- 

 tosis occurs during the periods of development mentioned is as 

 follows : 



First, there are in the development of the oogonia fewer cases 

 of mitosis to be found than one would a priori expect, and it 

 might be claimed that the number present is not sufficient to 

 account for the cell multiplication which must have occurred. 



Second, the arrangement of the nuclei in pairs suggests in 

 the light of the fact just stated that the cells may have arisen 

 by direct division. Two nuclei often occur close together but 

 separated more or less from their neighbors. 



Third, constricted and indented nuclei do occur in the early 

 stages of development of the oogonia; but they occur in my 

 material only in cases of imperfect fixation, so they lend very 

 little support to the amitosis view. 



Fourth, in the cleavage stages but one condition (unless ac- 

 cessory sperm nuclei might be confused with direct cell division) 

 has been seen in properly fixed material which might be inter- 



