FERTILIZATION 609 



and a small nucleus together in the formation of the pansporoblast. A 

 considerable portion of the life cycle of these organisms is therefore 

 passed in the haploid state. Naville also describes a similar diploid- 

 haploid cycle of four and two chromosomes for Myxidium incurvatum. 

 In this case there are more variations in the method of spore formation, 

 but reduction from four to two chromosomes occurs in the plasmodium, 

 as in Sphaeromyxa, and fertilization occurs between the two remaining 

 nuclei of the spore. 



Many variations of this process occur in other forms, but Chloromyxum 

 leydigi (Naville, 1931) is of particular interest, because of its two hap- 

 loid-diploid cycles. In this multinucleated plasmodium, the nuclei divide 

 by mitosis, showing a diploid chromosome number of four. Then a 

 heteropolar reduction division occurs, producing large and small nuclei 

 with two chromosomes each. Internal buds are formed, wherein large 

 and small nuclei fuse in pairs (first union). The difference in size of 

 these fusing nuclei makes this an anisogamous fertilization. Several divi- 

 sions of the fusion nuclei follow, each showing the diploid four chromo- 

 somes. The young plasmodium grows until the advent of spore forma- 

 tion, which is marked by the appearance of groups of four nuclei, two 

 large and two small, each with chromosomes again reduced to the hap- 

 loid two. The two small nuclei degenerate, while the larger two divide 

 twice, to form a group of eight which become enclosed in a wall. Of 

 the eight nuclei thus formed, six function in the formation of the spore 

 complex and the remaining two fuse in the second fertilization of the 

 cycle. This second fusion is comparable to fertilization in other forms, 

 but the first fusion of nuclei in the plasmodium is a secondary develop- 

 ment interpolated in the life cycle. Its significance is a matter of specula- 

 tion. The phenomenon is actually a double autogamy and is difficult 

 to harmonize with meiotic processes of either Metazoa or other Protozoa. 

 Little is known of chromosome behavior in other Cnidosporidia. 



Among the Actinomyxida and Microsporidia, autogamous fertiliza- 

 tion is said to occur in a manner broadly similar to that of the Myxo- 

 sporidia. One member of the Actinomyxida, Guyenotia sphaerulosa, has 

 been shown by Naville (1930a) to undergo chromosome reduction in 

 the second of three gametogenic divisions. In this case the development 

 of the pansporoblast occurs as described in the case of other Actino- 

 myxida. The two nuclei of the sporozoite divide mitotically, forming 



