206 



CYTOLOGY 



CHAP. 



cycle than is found in the Metazoa, where with the exception of certain 

 cases of parthenogenesis the haploid stage is represented by at most two 

 cell generations (p. 214). 



A closely similar account of the nuclear cycle of the Gregarine Dipio- 

 cystis schneideri is given by the same authors {loc. cit.). 



The third principal mode of nuclear multiplication in the Protista — 



by the intermediation 



a 



;? 



B 



c 



(in 



I 



jS5^ 



dd' 



ee 



cc 



of chromidia formation 

 — is supposed to be of 

 frequent occurrence. 

 The three forms, Mas- 

 tigella, Coccidium and 

 Arcella will serve as 

 examples. 



During the asexual 

 multiplication of Alas- 

 tigella vitrea (Gold- 

 schmidt, 1907) the nuc- 

 leus divides by mitosis 

 with well - developed 

 chromosome formation. 

 In gametogencsis, how- 

 ever, the gamete nuclei 

 are produced from the 

 nucleus of the gameto- 

 cyte by a very different 

 process (Fig. 87) . There 

 is a copious emission 

 of chromidia from the 

 nucleus into the cyto- 

 plasm. In the mass of 

 chromidia numerous 

 nuclei (up to two or 

 three hundred) are formed by aggregation of numbers of chromidia into 

 clumps. The nuclei thus formed then (in the case of the macrogamete) 

 divide by mitosis (with chromosome formation) at least once. Gold- 

 schmidt interprets this as a reduction division, as it results in only one 

 of the two daughter groups of chromosomes forming a functional nucleus, 

 the other degenerating into a body strikingly reminiscent of the Metazoan 

 polar body. 



The micro- and macrogametes unite in syngamy, the zygote nucleus 

 dividing by mitosis to introduce the asexual cycle with which we began. 



« 



f 



The chromosomes of Aggregata eberthi. They are designated a-f, 

 from the largest (a) to the smallest (/). (Dobell and Jameson, P.R.S., 

 igi5.) A microgametocyte nucleus in prophase, with six chromosomes ; 

 B, equatorial plate, and C, anaphase of same ; D, zygote nucleus in pro- 

 phase for its first division : E, equatorial plate of same division — homo- 

 logous chromosomes united into bivalents ; F, anaphase of same 

 (reduction division). 



