REPRODUCTION IN PROTOZOA 



141 



are in the ascendant. Reproduction occurs at the hmit of 

 growth, or sometimes when nutrition is checked. 



As it is by cell division that all embryos are formed from the egg, and 

 all growth is effected, the beginnings of this process are of much interest. 

 (a) Some very simple Protozoa seem to reproduce by what looks like 

 the rupture of outlying parts of the cell substance. (&) The production 

 of a small bud from a parent cell is not uncommon, and some Rhizo- 

 pods (e.g. Arcella, Pelomyxa) give off many buds at once, (f) Com- 

 moner, however, is the definite and orderly process by which a unit 



Fig. 65. — Diagram of the structure of Noctiluca. 



FL., Large locomotor flagelluin ; fl., small food-inwafting 

 flagellum ; M., mouth ; iV., nucleus ; P., central 

 protoplasm around nucleus, with granules and food- 

 particles ; R., a ridge in the "gullet" — a con- 

 tinuation of the mouth opening ; V.C., much vacuo- 

 lated general cytoplasm ; CU., cuticular pelUcle. 



divides into two — ordinary cell division, {d) Finally, if many divisions 

 occur in rapid succession or contemporaneously, and usually within a 

 cyst enclosing the parent cell, i.e. in narrowly limited time and space, 

 the result is the formation of a considerable number of small units or 

 spores. In the great majority of cases, each result of division is seen 

 to include part of the parent nucleus. 



A many-celled animal multiplies in most cases by 

 liberating reproductive cells — ova and spermatozoa- 

 different from the somatic cells which make up the " body." 

 A Protozoon multiphes by dividing wholly into daughter 



