318 THE EVOLUTION THEORY 



selves side by side (Fig. 83) and coalesce at the surfaces in contact, 

 both as to the spherical gelatinous envelope {A, G) and the proto- 

 plasm {jjr) itself, which branches in amoeboid fashion into the jelly. 

 The union becomes gradually complete, and the two animals form 

 a single sphere {B) with one cell-body. But the two nuclei {K) also 

 place themselves side by side (B), and though they do not actually 

 fuse, they form together, under the guidance of two centrospheres (0), 

 a single nuclear division-figure, which is obviously analogous to the 

 segmentation spindle of the fertilized egg. Then follows a division, 

 by means of which the chromatin substance of the nuclei of both 

 animals is divided between the two daughter-nuclei, and after this 

 has been accomplished the united individual again separates into two 

 independent Noctilucas {D). Although I have spoken here — that is, 

 in referring to the Protozoa — of chromosomes, I must immediately 

 add that these have not yet been seen with full clearness in Noctiluca 

 itself ; nothing more has been recognized than deeply staining thicken- 

 ings of the spindle fibrils, which move from the equator of the 

 nuclear spindle towards the pole. Since, however, in other Protozoa, 

 as, for instance, in the beautiful freshwater Rhizopod {Euglypha 

 alveolata), these thickenings of the nuclear spindle fibrils have been 

 clearly recognized as chromosomes, doubt on this point is hardly 

 justifiable. Apart from this, the assumption that each of the two 

 daughter-nuclei receives half the chromosomes of each of the con- 

 jugated nuclei rests on a secure basis, not only because otherwise the 

 whole process would have no meaning, but because the position of the 

 mitotic figure conditions this. Even the fact that the two conjugation- 

 nuclei lying side by side remain apart during nuclear division is not 

 without parallel; Hacker and Riickert observed it also in the seg- 

 mentation-nucleus of much higher animals, the Copepods, and it has 

 no eflTect in altering the process of division, but only proves that the 

 chromosomes of maternal and those of paternal origin in the com- 

 bination-nucleus remain independent — a fact the significance of which 

 I shall discuss later on. 



The process of conjugation occurs, in the same manner as in. 

 Noctiluca, in a freshwater Rhizopod, the well-known Sun-animalcule, 

 Actinophrys sol (Fig. 84), but in this case complete fusion of the two 

 nuclei takes place (Fig. 84, V) before the formation of the division- 

 spindle (VI, 82}), which, with the simultaneous division of the cell- 

 body, gives rise to two new individuals. The process in this case is 

 especially interesting, because Schaudinn has succeeded in observing 

 a maturation division (///, Rsj^, directive spindle) as well as in 

 demonstrating polar bodies [I-V, Rk). Thus the analogy with the 



