PHYSIOLOGY OF THE INVERTEBRATA. 407 



is followed by endogenous division. (4) Gemmation, as in 

 Vorticdla. 



The Infusoria ciliata multiply by division or by conjugation. 

 The first mode of reproduction is effected by a constriction of 

 the adult cell in a transverse direction. The cell ultimately 

 becomes divided into two portions which separate ; each 

 portion finally developing into separate organisms. 



Paramcecium bursaria conjugates in pairs when the anterior 

 ends of two individuals unite and remain united for five or 

 six days. According to Balbiani,* the nucleolus and nucleus 

 of each organism, at this period of their life-histories, become 

 converted into sexual organs. The nucleolus $ is converted 

 into an oval body which acquires a striated structure ; ulti- 

 mately it divides into two or four parts. These parts again 

 divide, giving rise to capsules containing rods which are 

 pointed at one end. These rods represent the spermatozoa 

 of higher forms. The nucleus 9 gives rise to bodies 

 analogous to ovules. The result of conjugation is the forma- 

 tion of cells which escape as young Paraincecia. During the 

 act of conjugation the two organisms, as already stated, are 

 always united together at their anterior ends ; in other words 

 at the apertures which form the mouth. " It has been 

 thought that this aperture must play the part of a sexual 

 orifice through which the two organisms in copulation effect the 

 exchange of reproductive matter ; it has also been suggested, 

 moreover, that a special sexual orifice is present close to the 

 mouth ; but these questions of structure are still doubtful." 



Balbiani's investigations have been confirmed by Claparede, 

 Lachmann, Kolliker, Stein, Biitschli, Griiber, and others. 



It should be borne in mind that in these low organisms the 

 nucleus of the cell is the all-important agent in producing 

 many physiological functions — without it, the above mode of 

 reproduction cannot take place. In fact, it has been stated 

 that "the nucleus plays the primordial role in the cell; if, to 

 use an old comparison of Aristotle's, w^e compare the proto- 



* Journal de la I'hijslokKjic, tome i (1S5S) 



