544 • BULLETIN OF THE 



There are probably at least two modifications of the process by which 

 the fibre thickenings are converted into a single nuclear structure. It 

 has been repeatedly shown, that in some instances the individual thick- 

 enings pass through a vacuolar stage, and that nucleolar bodies are found 

 in the vacuoles before their ultimate confluence, — the " multinuclear" 

 condition of the cell. In Limax I have been unable to detect such a 

 condition, and am therefore inclined to believe that the diff'erentiation 

 of nucleoli does not take place until after the fusion of the thickenings. 

 But this difference is not one of fundamental significance, since in cases 

 where clusters of nuclei are developed their confluence in some instances 

 regularly ensues much earlier than in others. Limax, then, only fur- 

 nishes one of the extreme examples, since here the confluence takes place 

 before the formation of nucleoli. The postponement of the fusion, 

 observed in so many cases, and the consequent presence of a number of _ 

 apparently unconnected vacuolar structures, no more warrant the con- 

 clusion that a multinuclear condition exists than does the earlier state, 

 when the nuclear substance consists of a group of more numerous fibre 

 thickenings. It is only a stage in the process of concentration into a 

 single nucleus, and these different phases under which it occurs only 

 serve to make this interpretation more reasonable. In all cases of a 

 spindle differentiation the processes are essentially the same ; there is 

 a fusion of the nuclear substance of the thickenings with the protoplasm 

 of the yolk, and the end result is a single nucleus. 



The proportion of the nuclear substance from the old nucleus, which, 

 by means of the '* thickenings," directly contributes to the formation of 

 the new nuclei, although approximately constant for a given stage in the 

 development of any given animal, is subject to wide variations from one 

 animal to another; and there exist even more extreme modification, 

 between remote cell-generations in the same individual. In Limax, 

 where the nuclei attain a large size, the proportion is very small, espe- 

 cially in the formation of the male pronucleus, if, as is probable, the latter 

 is initiated by a single spermatozoon ; somewhat greater in the star- 

 fish and the Hirudinea, for example, where the nuclei remain compara- 

 tively small. But in plant cells the proportion is often very large, and 

 in certain tissue cells, as Flemming has shown, almost a maximum. 



Whether there exist cases in which the old nucleus simply divides 

 without any metamorphosis, — without the least interchange of sub- 

 stance with the surrounding protoplasm, — and, if so, whether the condi- 

 tions prevailing in tissue cells present stages of transition between the 

 more direct and the more complicated methods of nuclear division, •- 



