LINNEAlf SOCIETT OF LONDON. XXVU 



It is now admitted that in tlie reproduction of unicellular organ- 

 isms, both in the animal and vegetable kingdom, the nucleus takes 

 an important part, and by its division as a primary act ushers in 

 the division of the rest of the protoplasm. Even in the cells which 

 form constituents of tissues the part played by the nucleus is alto- 

 gether similar, its division always preceding the division of the cell 

 itself. 



In quite a similar way does the nucleus behave in the ciliate 

 Infusoria. The non-sexual reproduction of the Infusoria by divi- 

 sion is perhaps universal. In such cases the division always begins . 

 by the spontaneous halving of the nucleus ; and this is followed by a 

 similar division of the surrounding protoplasm, exactly as in tl\e 

 ordinary simple cell. 



Another phenomenon in which the nucleus plays an important 

 part is named by Haeckel "spore-formation." Under this desig- 

 nation he comprehends all those cases in which, the idea of a previous 

 fecundation being rejected, the nucleus breaks into numerous 

 pieces, and each of these, apparently by becoming encysted in a 

 portion of the protoplasm of the mother body, shapes itself into an 

 independent cell, a so-called germ-globule (Keimkugel). Now this 

 is a true spore, just as much so as the spores, which arise quite in 

 the same way, in unicellular plants. The whole process is to be 

 regarded as a case of the so-called endogenous multiplication of 

 cells. 



Most authors, however, take a different view of the nucleus. Fol- 

 lowing Balbiani, they regard it as an ovary, and to the fragments 

 into which it breaks up they assign the significance of eggs ; while 

 the so-called nucleolus, which lies outside the nucleus, is believed to 

 be a testis in which spermatozoa are developed for the fecundation 

 of the eggs. 



We must bear in mind, however, that this " nucleolus" has been 

 hitherto found in but a disproportionately small number of species, 

 while the spermatozoal nature of the apparent filaments which have 

 been noticed in it has by no means been proved ; and we have 

 already seen that some observed facts, such as those adduced by 

 Biitschli, are opposed to the view which would assign to them the 

 nature of true spermatozoa. 



As Haeckel remarks, however, even though the so-called nucleolus 

 be really a testis fecundating the eggs or fragments derived from 

 the breaking up of the nucleus, this would afford no valid argu- 

 ment against the unicellularity of the Infusoria ; for precisely the 



