RHIZOPODS, ACTINOPODS, SLIME MOLDS, SPOROZOA 97 



These accumulated observations cover most stages in Plasmodium 

 development except for the sexual phases in the mosquito and the 

 young motile zygote. It appears that from early developmental 

 stages the parasite is enveloped by at least two unit membranes. 

 In the vertebrate erythrocyte it clearly is embedded in host cyto- 

 plasm, probably with no intervening host membrane. Feeding by 

 ingestion of host cytoplasm in the erythrocytic stage is established ; 

 in extraerythrocytic parasites further study is needed. The 

 formation of limiting membranes around developing merozoites 

 and sporozoites may perhaps proceed by invagination of sheet-like 

 extensions from the parent cell membrane, or it may occur by 

 neoformation of membranes within the cytoplasm. This question 

 has not been resolved, but in any event the increase in membrane 

 area is enormous. The fact that some species of Plasmodium 

 possess morphologically typical mitochondria while others lack 

 them poses a problem of considerable interest. Trager (1960) 

 recently has been able to maintain the bird parasite P. lophurae 

 apart from its host erythrocytes for a limited time, permitting a 

 significant study of nutritional requirements in this form and 

 offering a promising approach to the study of such matters as host 

 specificity. Metabolic studies on species with and without 

 identifiable mitochondria might, hopefully, be possible by these 

 means. 



Sporozoites and merozoites are supposed to. be motile, at least 

 briefly. Invasion of reticuloendothelial cells apparently is accom- 

 plished in part by the host's phagocytic activity (Meyer and 

 Oliveira Musacchio, 1960), but entrance into erythrocytes — 

 which has only rarely been observed in the light microscope 

 (Trager, 1960) — must be an active process. Enzymes are pro- 

 bably released to facilitate penetration but it seems as though some 

 mechanical activity must be involved. Garnham and colleagues 

 were tempted to suggest that the peripheral fibers seen by them in 

 sporozoites could be related to locomotion, while the paired 

 organelle was a glandule secreting lytic enzymes and the anterior 

 ring might function in attachment. Only the polar ring has been 

 described in any stage of the vertebrate cycle. All three of these 

 enigmatic features are suggestively similar to structures seen by 

 several authors in Toxoplasma and in Sarcocjstis (see below). 



Babesia is an organism parasitic in vertebrate erythrocytes and 



