FERTILIZATION 171 



example those of some Insects, Petromyzon, Selachians, Uro- 

 deles, Reptiles, Birds, and perhaps the toad and some Teleosts. 

 Sometimes a few, sometimes many, sperm thus enter the 

 ovum, but in any case only one of them ever takes any real 

 part in the actual processes of fertilization. The others, known 

 as accessory spermatozoa, may either remain quite inactive and 

 soon degenerate, or they may give rise to "vegetative" nuclei, 

 and perish after a brief period of activity. While active they 

 seem chiefly to be concerned in the preparation of the yolk for 

 ready absorption; they are then called merocytes. Rarely, if 

 ever, do the nuclei derived from accessory spermatozoa con- 

 tribute directly to the formation of any part of the embryo 

 proper. 



Apparently there is little specific adaptedness in the behavior 

 of the germ cells such that an egg and a sperm of the same 

 species tend to unite much more readily than do those of differ- 

 ent species. With some eggs any spermotozoon that is morpho- 

 logically capable of gaining entrance, can do so, apparently 

 about as readily as the specific sperm. The limitations here 

 are frequently due to the size of the sperm head as compared 

 w r ith the micropyle, or to the necessity for special perforating 

 mechanisms or powerful swimming movements in order to 

 penetrate the egg membranes, or the performance of appro- 

 priate reactions upon the part of the egg itself. As a rule the 

 eggs and sperm of a single species unite, because, as the result 

 of the breeding or spawning habits, only the sperm and ova 

 of a single species are associated in time and space in any con- 

 siderable numbers. When eggs are placed in a mixture of 

 equal quantities of two or more kinds of sperm, there seems to 

 be no appreciable selective fertilization, provided, as said above, 

 that both or all kinds of the sperm are able to enter the egg 

 at all. 



The ease with which "foreign" sperm may enter an egg is 

 affected hi many instances by chemical treatment of the eggs 

 and sperm; treatment with alkalies or with specific salts often 

 renders penetration of the sperm readily possible in cases 

 where normally it is difficult or impossible (Loeb, Godlewski). 



