FERTILIZATION 



191 



and often very complicated and the details cannot be given here. 

 (Many of these details, and references to the literature of the subject, 

 are given by Calkins, " Protozoology," New York, 1909). 



As a preliminary to the description of typical karyogamic union we 

 may refer to the very special form known as autogamy, which occurs 

 in many of the simplest Protista (e.g., Entamceba, Amoeba, some 

 Myxosporidia) . In autogamy there is really no fusion of cells at all; 

 the characteristic event is the separation of the nuclear chromatic 



FIG. 98. Autogamy in the Flagellate, Trichomastix lacertce. After Prowazek. 

 A. First nuclear division in the encysted form. B. The two nuclei completely 

 separated. C. First "reducing" division. D. Second "reducing" division. 

 E. Approach of the "reduced" nuclei. F. Fusion of the nuclei to form a single 

 nucleus (synkaryon). 



substance of a single cell into a number of separate bodies (Figs. 98, 99), 

 which become scattered through the cytoplasm as chromidia, or rather 

 as idiochromidia, for they are concerned in reproduction. After their 

 formation is completed these idiochromidia fuse, by twos, or sometimes 

 in larger groups, forming in effect "new" nuclei, or at any rate new 

 combinations of chromatic substance. These fused chromatin masses 

 then commonly move to the surface of the cell and are budded off 

 with small bits of cytoplasm, as small cells or spores (Fig. 99) (Schau- 

 dinn, Calkins) . Here then the nuclei which fuse together are the direct 

 derivatives of a single nucleus, and they remain within the same cyto- 

 plasmic mass throughout their formation and fusion. This might 

 readily be regarded as an extreme form of endogamy; it suggests the 

 roughly analogous process of the reentrance of the nucleus of one of 

 the polar bodies which occurs in a few of the parthenogenetic Arthropods. 

 Fertilization by autogamy is considered by some as a primitive method 

 of fertilization preceding all processes of gamete formation or cell fusions ; 

 others regard it as a derived condition in which the nuclei act preco- 



