FERTILIZATION BY ENDOGAMY 



149 



nuclei remaining (from one to twenty). Each of these daughter cysts 

 secretes a gelatinous envelope about itself, and the nucleus of each 

 divides by mitosis. This mitotic division is followed by division of 

 the cytospore into two daughter cells (cytospores No. 2), and in these 

 there are two successive nuclear divisions resulting in four nuclei. 

 Three of these nuclei degenerate ("polar bodies") and one remains as 

 a pronucleus. The cytospores of the second order next unite again, 

 reforming the cytospores No. 1, and the fertilization is completed by 



Fig. 63 



Endogamy in Actinospheriuni eichhornii. (After Hertwig.) ,-i, two gametes (cytospores 

 No. 2), resulting from the division of cytospore No. 1; B, both polar bodies are formed in 

 the right gamete, the second one forming in the left gamete; C, later fusion of the gametes, 

 the nuclei now uniting and the polar bodies being absorbed at p; D, young actinospherium 

 leaving cyst. 



fusion of the pronuclei. Thus, h\ a process of union of sister cells 

 (endogamy) fertilization is brought about after complicated matura- 

 tion processes (Fig- G3). 



Finally, in Paramecium aurelia, Calkins ('02) found that cells 

 removed by not more than eight or nine divisions from a common 

 ancestral cell would conjugate normally, and that such fertilized cells 

 were able to live through an entire cycle of cell generations (379 

 actuallv). Conjugation between closely related forms, therefore, is 

 quite as potent as between those of diverse ancestry. 



