I4 o THE EVOLUTION OF SEX. 



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 fertilization 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 

 portion. 



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



ciliated infusorians is re-established. 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 organization 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 observed in this particular case, he 

 was right in his contention that sexual union and fertilization really occurred in 

 the Protozoa. Balbiani's view has been for long scouted, and yet, with 

 renewed observation, naturalists have now come back to his conclusion. 

 Maupas willingly allows that Balbiani figured beautifully what he himself has 

 since reobserved and interpreted. 



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

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

 believe in their general occurrence. In three species of the slipper animalcule 

 (Param&cium), and in species of Sfylonichia, Leucophrys, Euplotes, 

 Onychodromns, Spirostomum, &c, the facts are as above stated. 



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

 ( Vorticella), because here the conjugating individuals are like ovum and sperm 

 in more ways than one. In some species — for example, V. monilata — 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 Carchesium polypinum, the divisions are equal, but 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 normal size, first to the 

 stalk, and then to the body (fig. p. 121). The accessory nuclear bodies divide 

 as usual ; the large individual ceases to feed, and hermetically closes its mouth, 

 like an ovum when fertilized. The small individual is gradually absorbed by 

 the larger, as sperm by ovum; and in an intricate but orderly fashion a mixed 

 nucleus results from the fusion of the para-nuclear elements of the two. The 

 adult then begins to feed, to divide, and so on, as usual. Here then there is (a) 

 incipient dimorphism, (b) absorption of smaller by larger, and (r) intimate 

 nuclear union, — facts which we have already emphasized in the fertilization of 

 multicellular animals. 



