FERTILIZATION BY ENDOGAMY 147 



cells" arise in the plasm of the adult organisms in the. same way as in 

 other myxosporidia, but the nuclei and with them the cell body of the 

 germinal area divide (Fig. 61, A, B, C). These propagative cells 

 later unite two by two, and are separated only by a thin cell wall, 

 which later disappears. Within this united mass the nuclei divide 

 until there are fourteen, as in spheromyxa; their formation differs in 

 some unessential details, but their fate is the same in both cases, two 

 germinal nuclei finally resulting which conjugate in the mature spore 

 (Fig. 61, D, /). 



Caullery and Mesnil ('05) have carefully described the process of 

 spore formation in spheractinomyxon, one of the actinomyxidae, an 

 aberrant group of myxosporidia named by Stole ('90). Here the 

 process is a little more complex than in the case cited above, but it 

 agrees in essence with that described by Keysselitz. The youngest 

 stages are found as intestinal parasites of the tubificid worm clitellio, 

 and are either uninucleated or binucleated. The observers are inclined 

 to believe that the uninucleated stage comes first and that it repre- 

 sents, possibly, the youngest form, or sporozoite, and that the binu- 

 cleatecl stage represents the first division of this nucleus. If this pos- 

 sibility is not well founded the fertilization process here must be taken 

 out of the present category. Whatever may be the origin of these 

 nuclei in the binucleated stage, they divide, and two of the first four 

 nuclei formed become somatic nuclei and are connected with the 

 formation of the cyst wall, within which the further processes take 

 place. With the division of the nuclei the cell body also divides until 

 there are sixteen independent, nucleated subdivisions. These unite 

 two by two, the process of fertilization being thus affected, and eight 

 spores ultimately result. The interpretation of this interesting case, 

 as Caullery and Mesnil point out, depends entirely upon the mode of 

 origin of the early binucleated stage. If these two nuclei represent a 

 plastogamic union of gametes, as Leger ( '04) believed to be the case 

 in an allied form triactinomyxon, then the process might be one of 

 exogamy, but, as Caullery and Mesnil contend, this would involve two 

 sexual processes in the life cycle, which seems improbable. The 

 subject certainly needs further study. 



The endogamous process in the mycetozoon Plasmodiophora 

 brassier is somewhat less complex than in the forms just described. 

 Here, as Prowazek ('05) has shown, the protoplasm breaks down 

 into many centres, each containing a sexual nucleus, and these centres 

 gametes fuse two by two, a spore wall being formed about each 

 copula (Fig. 62). 



In the majority of parasites the probability of endogamous fertiliza- 

 tion is readily apparent, and the fusion of gregarines, for example, 

 two by two, may be a union of cells from the same sporocyst or dif- 

 ferent sporocysts. In such cases it is impossible to state definitely, 



