DEVELOPMENT OP DISTICHOPORA VIOLACEA. 63 



We may go further than this^ though, and say that some of 

 Henking's figures, such as figs. 335, 336, and 337 of Lasius, 

 can only be interpreted on the supposition that the nucleus has 

 fragmented. The little clusters of chromatin granules, of very 

 irregular size and indefinite arrangement, that are here 

 figured scattered through the substance of the ovum, cannot be 

 considered to be the product of regular mitosis. 



It seems to be extremely probable that in the group of 

 insects we have a series of stages intermediate in condition 

 between regular mitotic division of the oosperm nucleus or its 

 immediate successors and irregular fragmentation. 



In Aphis (Will, 62) we may have regular karyokinesis at all 

 stages of the segmentation, the chromosomes being divided 

 into two equal halves at each division of the nucleus ; but in 

 Musca, in Lasius, and perhaps in several others in which the 

 earliest stages are passed through with great rapidity, the 

 nuclei fragment with greater or less irregularity. 



That the occurrence of karyokinesis is in some way dependent 

 upon forces manifesting themselves in the cell substance of 

 the ovum and acting upon the nuclei is rendered probable (1) by 

 the fact that in Aphis, where the nuclei divide by karyokinesis 

 in all stages, there is, as Will points out, a distinct aggregation 

 of protoplasm round the nuclei, and (2) by the fact that in 

 nearly all insects the karyokinetic figures of the nuclear 

 divisions that take place in the formation of the polar bodies 

 are much more regular and constant than they are in the early 

 stages of development. 



But I shall discuss this point and general significance of 

 mitosis in greater detail later on. 



That a similar process of fragmentation of the oosperm 

 nucleus may also occur in some Vertebrata seems to be probable 

 from the recent researches of Kastschenko (31) upon Elasmo- 

 branchs. It must be remembered that the early stages of the 

 development of Elasmobranchs and birds have been carefully 

 studied by numerous observers for the last twenty years, and 

 although the karyokinetic spindles in the developing blasto- 

 derm and its surrounding yolk have been described by nearly 



