SYNGAMY AND SEX IN THE PROTOZOA 153 



are sometimes markedly different in size (Doflein, 111). The 

 greatest amount of differentiation is seen in the order Peritricha 

 (p. 448), where microconjugants and macroconjugants can be dis- 

 tinguished. Each conjugant has a micronucleus and a macro- 

 nucleus. The macronucleus begins to degenerate, and finally dis- 

 appears completely. The micronucleus, on the other hand, en- 

 larges and divides by a simple form of karyokinesis (see p. 114, 

 supra). The division of the micronucleus is repeated twice as 

 a rule, but sometimes three times, and, as stated above, in one of 

 these divisions the number of chromosomes is halved in a great 

 many, possibly in all, cases. Of the four (or eight) micronuclei 

 thus formed, all but one represent reduction-nuclei which are 

 absorbed and disappear. The persistent micronucleus then divides 

 by equating division into two pronuclei, which may be distinguished 

 as migratory and stationary, respectively ; they sometimes exhibit 

 distinct structural differentiation. At this juncture the cuticle 

 of each conjugant is absorbed at the point of contact, and the 

 migratory pronucleus of each conjugant passes over into the 

 protoplasm of the other and fuses with its stationary pronucleus. 

 The gap in the cuticle is now repaired and the two individuals 

 separate, each " ex-conjugant " having a synkaryon constituted 

 by a fusion of one-eighth (or one-sixteenth) of its own original 

 micronucleus with the same fraction of the micronucleus of the 

 other partner. The synkaryon grows and divides into two nuclei, 

 one of which grows and becomes the macronucleus, while the other 

 remains small a,nd becomes the micronucleus, of the ex-conjugant, 

 which thereby becomes indistinguishable from an ordinary in- 

 dividual of the species, and proceeds to start on a course of vegeta- 

 tive growth and reproduction in the usual manner, until the next 

 act of syngamy initiates a fresh cycle. It has been observed that 

 the two ex-conjugants sometimes differ markedly in their capacities, 

 one of them multiplying much faster than the other. 



In the syngamy of Ciliata it is seen clearly that the macronucleus 

 represents effete vegetative or " somatic " chromatin, which is 

 eliminated bodily from the life-history of the organism, while the 

 micronucleus represents reserve generative chromatin from which, 

 after reduction, the entire nuclear apparatus is regenerated. The 

 remarkable feature in the syngamy of Infusoria is the manner 

 in which the conjugants remain distinct, and merely exchange 

 pronuclei (so - called " partial karyogamy "). Versluys (137), 

 following Boveri, derives this from an ancestral condition of iso- 

 gamic copulation that is to say, a condition in which the two 

 conjugants fused completely as gametes, both body and nucleus, 

 after which the zygote divided into two individuals ; on this view 

 the final division of the micronucleus which gives rise to the two 



