TJTRICULAll STRUCTURES. 169 



times a nucleolus is visible in both halves (as in Antho- 

 ceros)* Since the division of the nucleus exhibits 

 exactly similar phenomena to the division of cells, I con- 

 jecture that a parietal formation of the nuclear utricle 

 must be assumed in the former case like the parietal 

 cell-formation^ in the latter. 



The process just described may perhaps be borne out 

 by the facts known regarding the division of the nucleus 

 in animal-cells, j A large nucleolus appears in the parent- 

 nucleus, and divides ; the two secondary nucleoli retreat 

 from each other, and the nuclear utricle divides into two 

 secondary nuclear utricles, in such a manner that each of 

 them incloses a nucleolus. 



If we may draw conclusions as to the division of the 

 nucleus from the division of cells, where the appear- 

 ances are the same, the parietal formation of the nuclear 

 utricle depends on the following events : Two nucleoli 

 appear in the parent nuclear utricle ; the contents of this 

 separate into two portions, each of which incloses a nucle- 

 olus, and becomes clothed by a membrane ; the membrane 

 of each of the new nuclear utricles is in contact partly 

 with the wall of the parent-utricle, aud partly with that of 

 its fellow, whereby a septum is produced in the parent- 

 utricle. 



When the nuclei originate free in the cell-contents, the 

 circumstances are probably the same as those occurring 

 in the formation of free cells. It appears, namely, that, 

 first, a nucleolus is formed ; that a layer of mucilage is 

 deposited around this, and that the membrane of the nuclear 

 utricle is produced upon the surface of the mucilage. At 

 least the phenomena which can be observed in the forma- 

 tion of free nuclei allow of this explanation, which is 

 rendered probable by the analogy to free-cell formation. 

 I refer the reader, on this point, to the article " Free 

 cell-formation" in the preceding essay. 



* LOG. cit. pi. vi, fig. 38. 



f Loc. cit. p. 284. 



% Kolliker, Entwick. dcr Cephalopod., pi. vi, fig. 68. 



Page 95 et seq. 



