200 COMPARATIVE ANATOMY 



this act any two individuals of a swarm may unite together. 

 Every free schizogonous individual of Copromonas is therefore 

 a potential gamete, and since the members of any conjugant 

 couple are indistinguishable in shape and structure, and as 

 variable in point of size as are the individuals composing 

 the swarm, there is no recognisable distinction of sex, and the 

 gametes are isogametes. 



The sequence of events is as follows. Two gametes ap- 

 proach one another, come into contact by their anterior ends, 

 and soon become firmly united together. The flagella of 

 both continue to move actively, but one soon becomes shorter, 

 and at the end of a quarter of an hour or more is completely 

 withdrawn. Fusion of the bodies of the two gametes gradually 

 extends backwards, as shown in fig. 42, j and 4, until the 

 resulting zygote has the appearance of a single asymmetrical 

 individual (fig. 42, 5). After a time the body of the zygote 

 is remodelled so that, although noticeably larger, its contours 

 are exactly those of the ordinary free swimming form. The 

 nuclear changes accompanying these processes are of some 

 importance as an illustration of the rule that reduction of 

 the chromatic contents of the nucleus takes place prior to the 

 union of the nuclei in conjugation. No change is observed in 

 the nuclei of the gamete until a considerable degree of cyto- 

 plasmic fusion has been effected. The nucleus of each then 

 divides amitotically into two sub-equal portions (fig. 42, j), 

 and in each case one of the products of division, distinguish- 

 able from the first because of its paler colour in stained pre- 

 parations, rapidly degenerates and soon breaks up and is 

 absorbed into the cytoplasm. At a later stage (fig. 42, 4} a 

 second reduction of the chromatin of each nucleus is effected 

 by the extrusion of a small granule which similarly degenerates 

 in the cytoplasm. Though not certain, it is highly probable 

 that only one such granule is ejected from each nucleus, and 

 the process may be described as an asymmetrical or heteropolar 

 division of the nucleus, one of the division products being 

 much smaller than the other. When the bodies of the two 

 gametes are nearly completely fused, the two nuclei approach 

 one another and for some little time lie side by side with their 

 nuclear membranes in contact, but eventually the adjacent 

 membranes are absorbed and the two chromatin masses are 

 completely fused together. It seems that, just as was the 



