SEXUAL REPRODUCTION. I49 



(luring the process, while a genuine fusion of the two nuclei has also been 

 observed in permanent conjugation. 



In regard to the interchange of elements, there is considerable diver- 

 gence of observation. Joseph has noted what appears to be an interchange 

 of protoplasm ; Schneider has observed the exchange of nuclear elements ; 

 while Grul:)cr and Maupas, and Joseph as well, have, in their studies on the 

 union of ciliated infusorians, laid emphasis on an accessory nuclear body, 

 generally known as the "paranucleus." This body lies by the side of the 

 larger nucleus, and while the latter simply disrupts and dissolves away, or 

 is extruded without playing any important part, the smaller paranucleus 

 divides in a regular way, and with the results there is interchange between 

 the two individuals. 



According to Maupas, who has investigated the subject in most detail, 

 the para- or micro-nucleus is a "hermaphrodite" sexual element, of sole 

 importance in conjugation. The stages in the process of fertilisation are as 

 follows : — 



(i.) The para-nucleus increases in size. 



(2, 3.) It then divides twice, and eliminates certain corpuscles. 



(4.) This effected, it divides again, differentiating a male and female 



pro-nucleus. 

 (5.) In the next stage, the male elements of the two individuals are 

 exchanged, and the new male nucleus fuses with the original 

 female ])ortion. 

 (6, 7.) In two following stages, the nuclear dualism characteristic of the 

 ciliated infusorians is re-eslablished. The old large nucleus 

 (macro-nucleus) has broken up and been eliminated meanwhile. 

 (8.) Finally, the individuals, separating from one another, reassume all 

 their original organisation before beginning again to divide in the 

 usual fashion. 

 The union of the male and female nuclear elements in ciliate infusorians 

 was admirably figured by Balbiani so long ago as 1858 ; and though he does 

 not seem rightly to have interpreted what he oljserved in this particular case, 

 he was right in his contention that sexual union and fertilisation really 

 occurred in the Protozoa. Balbiani's view has been^for long scouted, and 

 yet, with renewed ol)servation, naturalists have now come back to his con- 

 clusion. Maupas willingly allow^s that Balbiani figured beautifully what he 

 himself has since reobserved and interpreted. 



The phenomena described by Maupas, as summarised above, have been 

 observed in towards a dozen ciliated infusorians, so that there is every reason 

 to believe in their general occurrence. In three species of the slipper ani- 

 malcule {Paraiii(rLiti//i), and in species oi StylonicJiia^ Leitcophrys, Eziplotes, 

 0)iycJiodro))nts, Spirostonnini, &c., the facts are as above stated. 



It is of interest to cite the facts in regard to the common bell-animalcule 

 ( ^; //rt'//a), because here the conjugating individuals arelikeovum and sperm 

 in more ways than one. In some species — e.g.^ V. inoiiilata — the adult 

 divides equally, to form two small individuals, which conjugate with those 

 of normal size. In V. microstoma there is again division into two, but the 

 products are of unequal size ; one is more male than the other. In the 

 nearly allied Carc/iesiiim polypvnim, the divisions are equal, Init they are 

 repeated twice or thrice. The result in all cases is the production of 

 minute individuals, which eventually attach themselves to adults of the nor- 

 mal size, first to the stalk, and then to the liody (fig. p. 129). The accessory 



