PHENOMENA ACCOMPANYING FERTILIZATION 539 



Aggregata eherthi as in Diplocystis the first division of the zygote 

 results in halving the number of chromosomes (Dobell, 1916). 

 The Hsemosporidia differ in that capsule-bearing sporoblasts are 

 not formed. Here the zygote grows to large size and the amphi- 

 nucleus divides repeatedly until myriads of sporozoites are formed. 

 In these tApes of Protozoa, therefore, metagamic activities involve 

 actual reproduction and reproduction here is a sequel to fertilization. 



Other groups of Protozoa differ widely in their metagamic activi- 

 ties and some types gives unmistakable evidence of ontogenetic 

 development. Thus zygotes of Foraminifera grow directly into the 

 more or less complex asexual generation (microspheric) . Here 

 the amphinucleus divides repeatedly while the cell divisions are 

 suppressed. Sunilarly in Phytomonadida the zygote after a resting 

 period di\'ides to form a colony of specific character and the meta- 

 gamic divisions are associated with cellular ditt'erentiations no less 

 regular in sequence than they are in many celled animals and plants. 



Other changes of a metagamic nature have to do with the clearing 

 up of accumulated substances in the cytoplasm. Zweibaum (1922) 

 finds that relatively large droplets of neutral fat which are charac- 

 teristic of vegetative phases of Paramecium are broken down prior 

 to conjugation while smaller droplets of another tjq^e accumulate. 

 Among these he was able to detect a larger amount of cholesterin 

 ester than normal and a great quantity of what he interpreted as 

 fatty acids. After conjugation these small drops disappear and 

 neutral fats reappear. A similar accumulation of fat-like droplets 

 and "lipoplasts" is described by Belaf (1922) in Actinoi)hrys sol 

 as characteristic of the copulating gametes and of the zygote, 

 but the accumulation breaks down and disappears with germina- 

 tion of the latter. ]Macrogametes of Coccidia have an analogous 

 store of cytoplasmic substances of the nature of lecithin which also 

 disappear during metagamic activities. 



There is some evidence, therefore, that specific products of 

 metabolism accumulate in cells of Protozoa prior to fertilization 

 and that these are utilized as are yolk substances of metazoon eggs 

 in the early metagamic activities. Their disappearance after fer- 

 tilization indicates that in this respect also, the general make-up of 

 the cytoplasm is reorganized. 



IV. PARTHENOGENESIS. 



Parthenogenesis may be briefly defined as the development of 

 an organism from an egg cell (or its equivalent, e. g., a ciliate) 

 which has not been fertilized. The phenomenon occurs spontane- 

 ously in a few animal groups and may be induced artificially in 

 eggs from animals of widely different phyla which usually undergo 

 fertilization before development. 



