SYNGAMY AND SEX IN THE PROTOZOA 151 



viduals, which copulate in pairs, and the uninucleate zygote grows up into the 

 multinucleate Pdomyxa. 



The conception of vegetative and generative chromidia has not been 

 accepted universally or without criticism. Hartmann, as pointed out above, 

 considers that many cases of generative chromidia are really the result of a 

 disruption of a polyenergid nucleus ; Awerinzew (47) is of opinion that, while 

 all Protozoa possess vegetative chromidia at some stage at least in the life- 

 cycle, generative chromidia are to be considered as a new acquisition, a hasten- 

 ing of the process of the formation of numerous gamete-nuclei ; Dobell (51) 

 puts forward a similar view. with regard to generative chromidia. With 

 regard to the latter criticism, it may be pointed out that nuclei may become 

 resolved into chromidia in order to undergo simple binary fission. With 

 regard to Hartmann's view, there is at present, at least, little evidence that 

 it is an adequate explanation of the many cases of formation of secondary 

 generative nuclei from chromidia known amongst the Sarcodina. The ques- 

 tion is discussed further below (p. 255). 



2. Syngamy and Reduction with Nuclei only. A very simple 

 example is furnished by the common Aetinophrys sol (Fig. 71), as 

 described by Schaudinn (129). Conjugation takes place between 

 two adult forms (macrogamy), which come together and become 

 enclosed in a common cyst. The nucleus of each individual then 

 divides by karyokinesis, and one nucleus of the pair thus produced 

 is expelled from the body and undergoes degeneration as a reduction 

 nucleus. The persistent nucleus of each individual then repeats 

 the process and forms a second reduction-nucleus. The nucleus 

 now remaining in each cell is the definitive pronucleus. The two 

 gametes now copulate, their pronuclei fusing to form the synkaryon, 

 after which the synkaryon divides by karyokinesis and the zygote 

 divides into two individuals which later escape from the cyst and 

 resume the free-living vegetative life. The course of syngamy in 

 Actinophrys is exactly similar to that performed by the two 

 " secondary cysts " derived from division of a " primary cyst " in 

 Actinosphcerium (see p. 138, supra). In both cases alike the nucleus 

 of the conjugants may be supposed to contain both vegetative 

 and generative chromatin mixed together. It is possible that the 

 vegetative chromatin is extruded from the nucleus in the form of 

 chromidia prior to the reducing divisions, but no elimination of 

 vegetative substance has been described. 



The last example of syngamy in Protozoa that need be con- 

 sidered specially at this point is that of the Infusoria, which have 

 been the subject of numerous investigations. These organisms 

 present the highest degree of specialization of the body-structure 

 and elaboration of the nuclear apparatus found in any Protozoa. 

 Their syngamic processes vary in detail to some extent in different 

 cases (see p. 448), but the whole process is essentially as follows 

 (Fig. 77) : Two individuals come together and adhere, placing 

 themselves side by side. The two conjugants may be similar in 

 visible constitution, or may differ to a greater or less extent, and 



