80 H. M. WOODCOCK. 



are not necessarily of opposite sex, and the encystiuent is only 

 temporary, the gametocytes becoming free later and giving 

 rise to markedly anisogamous gametes (example, Scliau- 

 dinnella).^ 



Now, the great success of the Gregarines is undoubtedly 

 due to their power of closely and permanently associating 

 during the period of formation of the gametes. Granting, 

 therefore, the possession by the parasites (in the first place) of 

 such a useful property, it seems most unlikely that they would 

 dispense with the same (as, according to Nusbaum, S ch au- 

 di nnell a has done) without very good reason. For in that 

 case we should have a tendency exactly the reverse of that to 

 which the prosperous development of the order has above 

 been attributed. Moreover, the primitive forms in the gut 

 (supposedly isogamous), being capable of movement, could 

 have no difficulty in associating; it is not easy to see, there- 

 fore, why they should adopt a less certain mode of sexual 

 reproduction. 



The coelomic Gregarines are very interesting in this 

 connection. Here, if anywhere, association is difficult of 

 attainment (cf. above, p. 67j. Hence, if anisogamy in 

 this order has been developed from an isogamous con- 

 dition, we might reasonably expect to meet Avith it here, 

 since by this means the necessity for permanent associa- 

 tion would be obviated. We find, on the contrary, how- 

 ever, that, speaking generally, it is these coelomic forms 

 which are most nearly or entirely isogamous. So far from 

 acquiring anisogamous gametes (comparable to those of 

 Pterocephalus or Stylorhynchus), they have, in many 

 cases, developed the condition of neogamy — that is to 

 say, they have been at pains to insure association. And in 

 the few cases where, if anywhere, permanent association can 

 apparently be dispensed with this is certainly not by the 

 development of anisogamy, but by the very unusual modifi- 



1 Nusbaum (loc. cit.) docs not endeavour to explain why, as is apparently 

 the case, tlie mah elements of this remarkable form arc non-motile, which 

 is hardly what one would expect according to his view of the course of evolution. 



