32:2 THE EVOLUTION THEORY 



nuclei across the briilj^'c t'ormed b}^ the union of the apposed surfaces 

 into the other animal (6, mi J ), so that it may form, by union with 

 the nucleus ^Yhich has remained there, a double nucleus (7), a structure 

 which corresponds to the segmentation nucleus of the ovum {cvpk). 

 From it there then arises by division a new macronucleus and a new 

 micronucleus, not usually directly, however, that is, not by a single 

 division, but through several successive nuclear divisions, into the 

 meaning of which I cannot here enter. Immediately after the union 

 of the two sex-nuclei the two animals sever their connexion with each 

 other ; each begins again to feed, and is subject to multiplication by 

 division just as it was before conjugation took place (8 and 9). 



Although the course of this remarkable process exhibits all 

 manner of differences in detail in different species, it is everywhere 

 the same in its essential feature, and this essential feature is un- 

 doubtedly the union of an equal quantity of the nuclear substance 

 of two animals to form a new nucleus. It is thus essentially the 

 same process which we have already recognized among higher animals 

 as ' fertilization.' The differences are of minor importance, and they 

 arise partly from the fact that the sex-cells of multicellular animals 

 are not independent self-supporting units, and partly from their 

 differentiation into ' male ' and ' female ' cells. The minuteness of 

 the sperm-cell, for instance, conditions its penetration of the ovum, 

 which is always much larger and passive, and also the thorough 

 fusion of its cell-body with the cell-body of the ovum. That this 

 difference has very little deep significance is best seen from the fact 

 that, even among Infusorians, there are forms in which the two 

 conjugating individuals are quite different, especially in size, and in 

 which the much smaller ' male ' animal fuses completely with the 

 much larger ' female,' and indeed bores its way into it after the 

 manner of a sperm-cell. This is the case among the bell-animalcules 

 (Vorticellinas) (Fig. 86), the conjugating pairs of which had been 

 observed long before our present insight into these processes had 

 been attained. Indeed, the facts had been interpreted as a kind of 

 * budding process,' the minute and differently shaped ' male ' animal 

 {mi), which at the time of conjugation is attached to the larger 

 ' female ' [ma), was regarded as its bud. This supposed bud, however, 

 does not grow out from the animal, but into it ! 



Thus we see here again that a differentiation of individuals as 

 males and females may occur among unicellular organisms, just as in 

 the sex-cells of higher animals and plants, and this proves to us once 

 more that all these differences of sex, whether in reproductive cells of 

 multicellular organisms, or in the entire multicellular animal or plant. 



