530 



BIOLOGY OF THE PROTOZOA 



ib) Gametic Meiosis (Wilson, 1925). -In the preceding section 

 instances of meiotic divisions subsequent to cell fusion were inter- 

 preted as due to stimuli mutually imparted to the conjugating 

 individuals. For this the protoplasm must be in a mature condition, 

 that is, with an organization conside^abl^' modified from^ that of 

 the young or immature organisms. In a later section evidence is 

 given which indicates that under proper conditions the stage is all 

 set for a similar all or none series of phenomena without however, 

 the stimulus of contact (see p. 540, endomixis). The latter con- 

 dition termed here gametic meiosis if accompanied by the cell 

 fusion of gametes, is characteristic of the majority of Protozoa 



Fig. 221.— Actinosphcerium eichhornii. A, two gametes ("cystospores No. 2") 

 resulting from the division of the same mother-oell; B, both "polar bodies" are 

 formed in the right gamete, the second one forming in the left gamete; C, the cell 

 bodies of the gametes have fused, and the nuclei are fusing; D, young organism leav- 

 ing cyst; p, p^ P^ "polar bodies." (After Hertwig.) 



in which fertilization is accomplished hy the fusion of cells. Unfor- 

 tunately the history of the chromosomes is known in but few cases 

 but there is scarcely a paper on the fertilization of Protozoa that 

 does not describe two rapidly-following divisions of the nuclei 

 prior to fusion, and these are called maturation divisions, and the 

 resulting nuclei "reduction nuclei." In Actinosphcprivm eichhornii 

 according to Hertwig (1S9S) the first evidence of the process is 

 encystment of the adult organism and excretion of waste matters 

 contained in the protoplasm. The nuclei are reduced in number to 

 from 5 to 10 per cent of the original number by fusion and absorption 

 in the protoplasm. The cell then divides into as many daughter 

 cysts as there are nuclei and these Hertwig calls cytospores No. 1, 



