CYTOLOGY OF REPRODUCTION IN ANIMALS 



125 



earlier in the threadworm, Ascaris (Fig. 85). In these latter cases, there- 

 fore, the cell entered is an oocyte in terms of nuclear condition but a 

 female gamete in terms of power to undergo impregnation. It will be 

 pointed out shortly that the union of the gametes may in turn be followed 

 so quickly by the first embryonal division that the latter process is begun 

 before the former is completed. This overlapping of the events com- 

 prising the general processes of meiosis, syngamy, and embryogeny com- 

 plicates the cytological study of reproduction in many animals. 



One of the first visible effects of syngamy in many eggs is an elevation 

 of the vitelline membrane which begins at the point of sperm entrance 

 and extends rapidl}^ over the egg, forming the so-called fertilization 

 membrane. This change and certain further alterations in the egg have 



Fig. 93. — Diagram of syngamy and cleavage in an animal that completes meiosis before the 

 entrance of the spermatozoon. 



been found to occur in Nereis even when the sperm is removed from the 

 egg immediately after it begins to penetrate. In some animals the 

 perivitelline space beneath the raised membrane is A\dde enough to allow 

 the rotation of the main body of the egg within it. After the fertilization 

 membrane has been formed, no more sperms enter. This is not due 

 simply to the presence of a mechanical barrier, but also to a new physio- 

 logical state in the egg cytoplasm, for sperms will not enter membraneless 

 fragments of eggs in this stage of development. 



The behavior of the sperm and egg nuclei (the pronuclei) during 

 syngamy is illustrated in Fig. 93. Soon after entering the egg, the sperm 

 nucleus commonly begins to enlarge and reveal a structure more like 

 that of an ordinary nucleus. Meanwhile the pronuclei approach each 

 other and meet. By this time the sperm nucleus has often, though not 

 always, assumed a size and structui-e aljout like that of the egg nucleus. 

 The two now proceed to fuse. As they do so both may have a metabolic 

 type of structure, the resulting diploid nucleus later entering prophase^ 



