272 THE SPOROZOA 



Comparing these two formulae, 1 it is seen that the main 

 differences between the two types are seen in two points. First, the 

 female gametocyte in Coccidia gives rise only to a single female 

 gamete, instead of to a number of them, and consequently there 

 is only a single zygote, and a certain number of male gametes are 

 wasted ; secondly, the zygote of the Gregarine becomes a sporo- 

 blast, and ultimately a spore, but the zygote of the Coccidian 

 becomes the oocyst, and gives rise to a number of sporoblasts, each 

 of which becomes a spore. 



The facts stated above have led some authors to abandon the 

 obvious comparison of spore to spore and cyst to cyst in Grega- 

 rinida and Coccidiidea respectively. Taking the zygote as the 

 fixed point, so to speak, in both types of development, it has been 

 urged that the product of zygosis should be strictly homologised 

 and that therefore the Coccidian oocyst should be compared with 

 the Gregarine sporocyst. From this basis of comparison the 

 Coccidian sporocyst is something not represented in Gregarines, 

 and similarly the Gregarine cyst is without parallel amongst 

 Coccidia. This interpretation of the homologies seems to raise 

 more difficulties than it solves, and we shall attempt to show that 

 the facts can be interpreted differently, and in a manner at once 

 simpler and more natural. 



There is one point in which the two life-cycles differ, which is 

 not shown by the formulae given above, but which is of crucial 

 importance. In Gregarines the two sporonts become enveloped in 

 a common cyst before they give rise to gametes, and the entire 

 process of zygosis goes on within the cyst. In Coccidiomorpha 

 the zygosis takes place between free gametes, which become 

 encysted after the process is complete. The rare instances in which 

 an oocyst is secreted by the female gamete before fertilisation, as 

 in Coccidium proprium, etc. (p. 227), is not really an exception to 

 this rule, since here also the male gametes are free, and a micro- 

 pyle is left for their entry into the oocyst. 



The relation of the encystment to the zygosis is probably the 

 clue to the solution of the problem, and affords a means of tracing 

 a simple phylogenetic origin for the two divergent types of life- 

 history. As an ancestral condition, common to all Telosporidia, 

 we may assume a type in which the gametocytes each formed a 

 number of gametes, as in Gregarines, these gametes, however, being, 

 like those of the Coccidia, free, that is to say, not enveloped in 

 any cyst. The trophozoites of this ancestral form were probably 



1 The formulae of the life-cycles are simplified, but not modified for purposes of 

 comparison, in those cases iu which the tposoblasts are transformed into gymnospores 

 or so-called sporozoites, as iu Aggregata amongst Gregarinida and Eimena (LegereltuJ 

 amongst Coccidiidea. In such cases, instead of "n Sporoblasts-^-w Spores xmn 

 Sporozoites," we must write " n Sporoblasts-^n Gymnospores " ; or, for Haemosporidia 

 or Aggregate, "n Sporoblasta xmn Gymnospores (Sporozoites)." 



