27G BIOLOGY OF THE PROTOZOA 



of gametes, such as those of Ophryocystis. An interesting observa- 

 tion by Dogiel (1923) on the parasitic ciliate (Cycloposthium bipal- 

 matum and in other Ophryoscolecidae as well (Dogiel, 1925) lends 

 some support to this theory. Here gametic nuclei are formed as in 

 other ciliates; one of these nuclei, the migrating nucleus, develops 

 a tail and, like a spermatozoon, makes its way through the mem- 

 brane of the peristomial region of the mother-cell, and into the 

 external chamber formed by the mode of fusion of the two gamonts 

 (Fig. 141). From this chamber it enters the other gamont by way 

 of the mouth and ultimately meets and fuses with the stationary 

 nucleus of this gamont. 



(6) Isogametes and Anisogametes. — The term copulation as used 

 in connection with the Protozoa refers to total and permanent 







--'J' / 



Fig. 141. — Cycloposthium bipalmatum. Conjugating individuals with spermatozoon- 

 like wandering nucleus. (After Dogiel.) 



fusion of gametes. Of these there is the greatest variety of struc- 

 tures and differences in different types of Protozoa. In very few 

 cases of isogametes do we find copulation between individuals 

 whose differentiations are not expressed by morphological char- 

 acteristics. In such types the individuals differ little if at all from 

 the ordinary vegetative forms except in a physiological sense. 

 Plastogamy or casual cell fusion is easily mistaken for such holo- 

 gamic copulation and descriptions of so-called fertilization proc- 

 esses in testate and in naked rhizopods, in Heliozoa and in different 

 types of flagellates are open to criticism on this ground. In the 

 case of Helkesimastix faecicola and H. major (Woodcock and Lapage, 

 1915, and Woodcock, 1921) the evidence, from observations on 

 living cells, seems to indicate that copulation of these flagellates 

 does occur, but even in these cases the interpretation is not above 

 criticism in the absence of cytological confirmation. 



The majority of isogametes show morphological characteristics 



