242 



BIOLOGY OF THE PROTOZOA 



nuclear division, but without cell division, development of the former 

 being indicated externally by the formation of a many-chambered 

 shell. Similarly in the Mycetozoa the zygote formed by ameboid 

 or flagellated gametes develops into a Plasmodium by cell fusions 

 and nuclear divisions. 



In the Sporozoa the zygotes, formed by union of similar gametes 

 (isogametes) or of dissimilar gametes (anisogametes) undergo a 

 variable number of metagamic divisions, three in the majority of 

 Gregarinida and two or more in the Coccidiomorpha. The end- 

 result of such metagamic divisions is the formation of two or more 

 similar sporozoites which are entirely different from the adult indi- 

 viduals and undergo a more or less complex development. When 

 they are introduced into a new host the sporozoites are liberated 



Fig. 126. — Development of a polycystic! gregarine (schematic) . n, nucleus of host cell ; 

 p, parasite. (After Wasielewsky.) 



from their capsules, or introduced naked into the blood by some 

 intermediate host. They make their way to the definitive site of 

 parasitism, penetrate into cells and begin their development. In 

 the simpler gregarines only the young stages are passed in such host 

 cells and growth is not accompanied by any marked structural 

 differentiations. In the polycystid gregarines the parasite never 

 becomes entirely detached from its host cell until it is fully mature 

 and de-differentiation begun by the loss of the attaching organ 

 (epimerite). With its growth the body becomes differentiated into 

 an anterior chamber (protomerite) and a nucleus-holding posterior 

 chamber (deutomerite) and in the different species these three 

 portions of the cell become variously ornamented and specialized. 

 The epimerite particularly becomes modified in different ways that 

 are useful for purposes of anchorage (see p. 536). It may be a mere 



