No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 52 1 



spindle and centrosome of Ascaris, but might it not be com- 

 pared to the nucleolus alone, or to the nucleolus plus centro- 

 somes of the metazoan cell ? However, the significance of 

 most protozoan " nucleoli " is very problematical. (Cf. the later 

 observations of Lauterborn, '95a.) 



9. Ontogenetic Origin of the Nucleolus. 



Very few observations have been made to determine the 

 mode of origin of the nucleolus, though there are numerous 

 hypotheses intended to explain it. We may leave aside, for the 

 time being, its mode of reappearance in the daughter-nuclei 

 after nuclear division, since a special section will be devoted to 

 that subject. 



In order to determine the mode of origin of the nucleolus in 

 resting stages of nuclei, I have studied those cells in which at 

 first no nucleolus is present, but which after a certain period of 

 growth acquire one. Objects well adapted for such investiga- 

 tion are the ova of the nemerteans and the mesenchym cells 

 of Cerebratidus. For details of these processes the reader is 

 referred to the observations. 



In the ova of the nemerteans the nucleoli at the time of their 

 first appearance are always in close contact with the nuclear 

 membrane ; this is also the case for the mesenchym cells of 

 Cerebratuhis , and probably for the paranucleoli of the ova of 

 Rodalia. In all these cells the nucleoli only then leave the 

 periphery of the nucleus and wander towards its center, after 

 the nucleus has increased more or less in size. There is only 

 one explanation for the peripheral position of the nucleoli at the 

 time of their first appearance, namely, that their substance is 

 extranuclear in origin. This process of formation has already 

 been discussed in detail for the several cells, and it is not 

 necessary to repeat here all the detailed observations on which 

 the main deduction is based. If the nucleolar substance were 

 a secretion of the nucleus, as Hacker ('95) assumes, how would 

 this assumption explain the strictly peripheral position of the 

 nucleoli when they first arise .' For on Hacker's hypothesis 

 we should expect the supposed nucleolar secretions to be de- 



