610 FERTILIZATION 



two large central germinal cells which ultimately give rise to eight male 

 and eight female gametes respectively, and two smaller peripheral cells 

 which divide again to form the four enveloping cells of the cyst wall. 

 Of the three gametogenic divisions, the second reduces the chromosome 

 number from the diploid four to the haploid two. The gametes are 

 differentiated on the basis of size, the male gametes being smaller than 

 the female. The eight microgametes unite with the eight macrogametes 

 to form eight zygotes and reestablish the diploid condition. 



The origin of the two nuclei in the sporozoites is not known, but it 

 is presumably by division of an original single nucleus. As the male and 

 female gametes are produced by a single original sporozoite, it may be 

 regarded as a hermaphroditic animal. This condition is rare in the Proto- 

 zoa except in ciliate conjugation, in which the two pronuclei produced 

 by one conjugant behave differently and in a few cases are morphologi- 

 cally differentiated (see p. 622 below). 



While a number of other life cycles of Cnidosporidia have been 

 worked out, knowledge of the fertilization process is fragmentary and 

 data on chromosome behavior in meiosis are almost entirely lacking. 

 Descriptions of "reduction" generally refer to the loss of chromatin 

 by the degeneration of some of the nuclei which are sisters of the func- 

 tional pronuclei. In other cases the extrusion of the karyosome is inter- 

 preted as reduction. Such loss of nuclear elements is not to be confused 

 with reduction in the chromosome number from the diploid condition 

 to the haploid. It is possible that the two processes are similar in func- 

 tion, but until such time as that is demonstrated, we are not justified 

 in assuming that one is the equivalent of the other. 



Autogamous fertilization has also been described for a few ciliates. 

 According to Buschkiel (1911), the parasitic form Ichthyophthirius 

 niultijiliis becomes encysted and the micronucleus divides twice produc- 

 ing four, of which two degenerate while the remaining two fuse au- 

 togamously. Fermor (1913) described a reorganization process within 

 the cyst of Stylonychia pustiilata, wherein the old macronuclei degenerate 

 and the micronuclei fuse and produce a new nuclear complex. The evi- 

 dence in support of these two cases is not conclusive. 



Diller (1936) has recently given a detailed account of autogamy in 

 Faramecium aurelia. The process is similar to conjugation except that 

 there is no pairing nor cross-fertilization. The macronucleus disintegrates 



