8 ORGANIZATIOX AXD LIFE-HISTORY OF PROTOZOA 



typically in the parasitic Sporozoa, and is known as schizogony. Usually 

 there is a residue which does not participate in the formation of the buds: 

 it is discarded as a residual body which quickly disintegrates. 



During the life-history of many cells a sexual process occurs from 

 time to time. The advantages gained from such a process, which is 

 called synga7Mj, are far from being clearly understood. In its simplest 

 form it consists in the complete union of two cells and fusion of their 

 nuclei. The uniting cells are known as gmnefes, and the single cell resulting 

 from the union is a zygote. The zygote proceeds to multiply by binary 

 or multiple fission. 



The process of syngamy must be distinguished from another type of 

 union which sometimes occurs. Two or more cells may fuse to produce 

 a multinucleate cytoplasmic body known as a plasmodiiitn. In this 



/ ■ .- - 



■■^..^' ^ \ ,,*: * / 



If ^ 





A B 



Fig. 1. — Diagram of Cells. (Original.) 



A. Metazoan cell. The cytoplasm contains a centrosome and a nucleus with a nucleolus. 



B. Protozoan cell {Entamoeba). The cytoplasm, differentiated into ectoplasm and endoplasm, 



contains a nucleus with central karyosome and numerous food vacuoles. No centrosome is 

 visible. 



manner plasmodia containing many hundreds of nuclei may be formed. 

 The nuclei show no tendency to unite with one another, as they do in 

 syngamy, and after the plasmodial phase has existed for some time 

 segmentation into. uninucleate cells takes place. 



The typical cell, wherever it occurs, consists of the two essential parts 

 — cytoplasm and nucleus (Fig. 1). Each of these is a mixture of 

 substances of highly complex chemical constitution, the reactions of which 

 produce the phenomena characteristic of living matter. The cytoplasm 

 appears to be made up of at least two substances, one of which is suspended 

 in the other in the form of an emulsion. The nucleus, which is limited 

 by a nuclear membrane, consists of a substance called nuclear sap, which 

 occupies interstices in a more solid material. The latter, when viewed in 



