420 



BIOLOGY OF THE PROTOZOA 



come from sporoblasts; sporoblasts from sporocysts; sporocysts from 

 zygotes ; zygotes from fusion of gametes ; gametes from gametocytes, 

 these from gamonts; gamonts from agametes; agametes from aga- 

 monts and agamonts, originally, from sporozoites. 



There are thus two complete cycles in the life history of a typical 

 sporozoon, an asexual and a sexual cycle. There are many varia- 

 tions in different types and few life c\cles conform exactly with 

 that of FAmeria. In the Eugregarines for example, the asexual 

 cycle is entirely eliminated, the sporozoite developing directly into 





Fig. 179. — Laiikesteria ascidice. Young sporozoites enter epithelial cells (A, B, C) 

 and grow directly into gamonts (D); two of these unite in pseudoconjugation (E), and 

 each forms gametes after repeated nuclear divisions (F, G, H). The gametes fuse 

 two by two (/, J, K,) and the zygotes undergo three metagamic divisions, forming 

 eight sporozoites (L to O). The parent cells degenerate and the sporocysts are filled 

 with sporoblasts, each with eight sporozoites. (After Siedlecki.) 



a gametocyte. In gregarines also we find a curious process which 

 recalls the phenomenon of conjugation in the Ciliata, but which in 

 Adelea is very similar to fertilization in VorticeUa. It is termed 

 pseudoconjugation. Two individuals come together side by side 

 or end to end and an envelope is secreted which encloses both indi- 

 viduals. This envelope becomes the sporoc\'st membrane. Each 

 indi\'idual now forms a large number of gametes and those from one 

 individual fuse with the gametes from the other individual and a 

 multitude of zygotes is formed. The actual fertilization membrane 



