REPRODUCTIVE ELEMENTS IN APUS AND BRANCHIPUS. 267 



or indirectly to the number of the chromosomes by all the 

 more recent investigators, and this factor in their origin (in 

 Bran chi pus) fully bears out the assumption that they are the 

 visual expression of the primary constitution of the cell to which 

 they belong. Nor is this all; for if we believe, as we have 

 every reason to believe, that the character of the nucleus is the 

 determining factor of the nature of the cell's activity, that 

 curious variation in the number of the chromosomes in cells of 

 closely allied species would be more intelligible ; for although 

 the frothy structure of the nucleus might be actually or closely 

 similar, a very slight difference in the cellular dimensions 

 would, provided the foam structure remained the same, 

 materially alter the number of the spaces between the globules, 

 and consequently the number of the chromosomes. It is at 

 the same time apparent that this number, as well as the 

 general nuclear characteristics, oscillate within narrow limits 

 for the same species.^ 



It is interesting to note in this connection that the characters 

 of nuclei, in Arthropods and Annelids, have much in common. 

 They nearly all present the peculiar ball-like chromosomes 

 during metamorphosis, just as they tend to form a reticulate 

 nucleus when at rest. In fact, we might say such nuclei 

 constitute an Annelidean nuclear type. 



Again, the characters of the Mammalian nuclei are very 

 constant, but they nevertheless differ in minor details even 

 from those of the Amphibia. In fact, the difference between 

 these two latter is as small as that between them both, and the 

 former, is great. The comparative study of nuclei is well 

 worthy of more minute attention; suffice it, however, at the 

 present moment to point out that such generalisations would 

 have weight in our conceptions of heredity. 



Of the regular occurrence of a peculiar intra-nuclear fusion 

 in Branchipus the appearances leave no doubt, or that it is 

 primarily instrumental in bringing about the conversion of the 



^ In connection with this see Valentine Hacker, "Die heterotjpische 

 Kerntheilung im Cyklus der generation Zellen," ' Berichteder Naturforschen» 

 den Gesellschaft zu Freiburg,' Bd. vi, 1892, pp. 160—188, 



