598 FERTILIZATION 



somes. Concerning the origin of the secondary nuclei, Le Calvez states 

 that he "has not been able to discover the chain of processes which, from 

 the disintegration of the vegetative nuclei, lead to the formation of a 

 well defined micronucleus" (secondary nucleus). He believes that the 

 hypothesis of generative chromatin ought to be completely abandoned. 



In the Actinopoda, sexual reproduction has been reported for both 

 Radiolaria and Heliozoa, but in only two forms has the process been 

 reliably described. The classical case is that of Actinosphaerium eich- 

 hornii (Hertwig, 1898). The multinucleated vegetative individual forms 

 a "mother cyst" and absorbs all but a few (up to 20) of its nuclei. The 

 cytoplasm divides into as many primary cysts (cytospore number one) 

 as there are nuclei. Each primary cyst divides into two distinct secondary 

 cysts (cytospore number two), the nuclei of which undergo two succes- 

 sive "reduction" divisions, resulting in one pronucleus and two "polar 

 bodies" each. The matured secondary cysts reunite with their sisters as 

 gametes, and the nuclei fuse to complete fertilization. This is obviously 

 a type of autogamy. Hertwig's claim that in both reduction divisions the 

 chromosomes (numbering between 120 and 150) are divided in the 

 metaphase seems open to question. If this were true, the divisions would 

 not be reductional in character, so that the chromosome number would 

 have to be reduced in some other manner than the usual gametic meioses. 



According to Schaudinn (1896), Actinophrys sol undergoes isoga- 

 mous macrogamy, or hologamy. He stated that two full-grown similar 

 individuals come together and form a common cyst. The nucleus of 

 each divides twice, and at both divisions one nuclear product degenerates 

 and is expelled. The two cells, with their matured pronuclei, then fuse. 

 The resulting zygote soon divides into two individuals, which later es- 

 cape from the common cyst as vegetative animals. 



The more recent and detailed investigations of Belaf (1923) have 

 demonstrated in this species a type of sexual activity similar in many 

 respects to that described by Schaudinn, except for the significant differ- 

 ence that the two original gametocytes within the cyst are sister cells, 

 since they are derived by a progamous division of the original gamont 

 (Fig. 142). The process, therefore, is a type of autogamy (pedogamy) 

 similar to that occurring in Actinosphaerium, except that in the latter 

 case the palmella produces several pairs of sister gametocytes. Incipient 



