DEVELOPMENT OF ELASMOBRANCH FISHES. 19 



bearing on the behaviour of the nuclei in as objective a manner 

 as possible. 



My observations are not as complete as could be desired; 

 but, taken in conjunction with those of other investigators, they 

 appear to me to point towards certain definite conclusions with 

 reference to the behaviour of the nucleus in cell-division. 



The most important of these conclusions may be stated as 

 follows. In the act of cell-division the nuclei of the resulting 

 cells are formed from the nucleus of the primitive cell. 



This may occur; — 



(1) By the complete solution of the old nucleus within the 

 protoplasm of the mother cell and the subsequent reaggregation 

 of its matter to form the nuclei of the freshly formed daughter 

 cells, 



(2) By the simple division of the nucleus, 



(3) Or by a process intermediate between these two where 

 part of the old nucleus passes into the general protoplasm and 

 part remains always distinguishable and divides ; the fresh 

 nucleus being in this case formed from the divided parts as well 

 as from the dissolved parts of the old nucleus. 



Included in this third process it is permissible to suppose 

 that we may have a series of all possible gradations between 

 the extreme processes 1 and 2. If it be admitted, and the 

 evidence we have is certainly in favour of it, that in some 

 cases, both in animal and vegetable cells, the nucleus itself 

 divides during cell-division, and in others the nucleus com- 

 pletely vanishes during the cell-division, it is more reasonable 

 to suspect the existence of some connection between the two 

 processes, than to suppose that they are entirely different in kind. 

 Such a connection is given by the hypothesis I have just proposed. 



The evidence for this view, derived both from my own 

 observations and those of other investigators, may be put as 

 follows. 



The absolute division of the nucleus has been stated to 

 occur in animal cells, but the number of instances where the 

 evidence is quite conclusive are not very numerous. Recently 

 F. E. Schultze ^ appears to have observed it in the case of an 

 Amoeba in an altogether satisfactor}^ manner. The instance is 



1 ArcMv f. Micr. Anat. xi. p. 592. 



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