PAR THENOGENESIS 



In some eggs, e.g., of insects, the first nuclear divisions ensue 

 without cytoplasmic cleavages, whilst in others, e.g., those 

 of selachians and of birds, later mitoses proceed without 

 the splitting up of the cytoplasm. In certain eggs experi- 

 mentally treated, some kind of development, abnormal 

 however, can go as far as a swimming form without cyto- 

 plasmic cleavage. Hence, although development never 

 completes itself without the earlier or later breaking up of 

 the cytoplasmic mass of the egg into cells, we can not cate- 

 gorically define the process of development as related to 

 the sundering of the cytoplasm. Therefore the end of the 

 fertilization-process we define by the appearance of the 

 mitotic figure rather than by the first cleavage of the 

 cytoplasm.^ 



Unfortunately, we have neglected to make nice studies 

 on the differences in the development of two sets of eggs 

 from the same animal, one set fertilized and the other 

 treated with some means of inducing parthenogenesis — 

 such differences, for example, that would reveal themselves 

 in the size of cells and in the time and the place of their 

 appearance in an egg whose cell-lineage is known, that 

 is, one in which the size, order of appearance and loca- 

 tion of the cells with reference to each other are definite 

 and invariable. Some differences, as rate of cleavage, we 

 know; others, indicated for the earlier stages of develop- 

 ment, are doubtless due to the fact that the experimental 

 treatment never quite duplicates the action of the sperma- 

 tozoon. But even if we assume that fertilized and par- 

 thenogenetically developing eggs, both in the induced and 



^ / say mitotic figure because the occurrence of amitotic nuclear 

 division in developing animal eggs has been doubted. If we include 

 the -possibilitv of development with amitotsis m our definition., we 

 substitute in the thesis nuclear division for the term, establishment, 

 or appearance, of the mitotic figure. 



241 



