THE SPOROZOA 



entirely intracellular, and the gametes formed by them were prob- 

 ably differentiated into active male gametes and passive female 

 gametes. Nevertheless, the male gametes must often have failed 

 to find the female gametes, which would then have to develop 

 parthenogenetically, or to perish unfertilised. From this hypo- 

 thetical ancestral stage the state of things existing in both 

 Gregarines and Coccidiomorpha may be derived by simple processes 

 of adaptive improvement and specialisation. 



In the Gregarines, with the acquisition of an intercellular 

 trophic stage, it became possible for the gametocytes to associate 

 and become encysted together, so that the gametes are formed in 

 a confined space, and there is no possibility of the male gamete 

 failing to find the female. In correlation with this condition the 

 differentiation of the gametes becomes less important, and, as Leger 

 has pointed out, conjugation takes place prematurely between 

 immature gametes, a condition which, carried further, has probably 

 led to the complete isogamy of such forms as Monocystis. 



In Coccidia the trophozoites remain intracellular, and hence 

 the gametocytes are usually kept apart, though occasionally 

 premature association takes place (Adelea, etc.). Correlated with 

 this state of things, a very great specialisation of the gametes is 

 brought about. In the female gametocyte the process of multipli- 

 cation to form gametes is in abeyance, and each female gametocyte 

 becomes a female gamete after elimination of nuclear substance. 

 The male gametes, however, are produced usually in large numbers 

 from the gametocyte (only when there is precocious association 

 of gametocytes is the number of male gametes reduced), and the 

 gametes themselves are of a highly specialised type. Thus the 

 probability of a male gamete finding a female is very great, even 

 if not a certainty, as in the Gregarines. Immediately after fertilis- 

 ation the zygote divides to form the sporoblasts, which may be 

 compared to those of Gregarines by supposing that the process of 

 multiplication by which the gametocyte of the Coccidia gave rise 

 primitively to a number of female gametes has not been com- 

 pletely suppressed, but merely deferred until after the process of 

 zygosis. 



From this point of view the female gamete of the Coccidia must 

 be compared not to a single female gamete of a Gregarine, but to 

 the whole number of those produced from a female gametocyte in 

 the latter, the actual process of cell-division being temporarily 

 arrested. This interpretation receives the strongest support from 

 some remarkable observations recently made by Schaudinn [51] 

 upon the life-cycle of the Coccidian, Cydospom caryolytica, parasitic 

 on the mole. In this form, as described above (p. 225), the 

 nucleus of the macrogamete normally throws off two " reduction- 

 nuclei" which degenerate, and the reduced pronucleus then 



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