THE MORPHOLOGY OF FERTILIZATION 55 



outflow of protoplasm which is the fertilization cone; 

 that then by coagulation in the cone the surface tension 

 of this region rises until it is overflowed by the surround- 

 ing protoplasm and sinks into the interior. The much 

 more rapid penetration in other forms can be under- 

 stood on similar principles. 



In Nereis we have seen that the head of the sper- 

 matozoon alone enters the egg. The tail and middle 

 piece remain outside. This is exceptional; in most 

 cases the entire spermatozoon enters, as for instance 

 in nematodes, Crustacea, mollusks, some insects, am- 

 phibia, and mammals (Fig. 4). In some sea urchins, 

 according to descriptions of various authors, the tail is 

 left outside, but the middle piece enters with the head. 

 The middle piece and tail represent cytoplasmic elements, 

 and the head is mostly nuclear material; it would appear 

 from the case of Nereis that the latter is sufficient of 

 itself for the subsequent events of fertilization; the middle 

 piece and tail are concerned primarily in accessory func- 

 tions of fertilization, such as locomotion. In the writer's 

 opinion this is their only necessary function, and their 

 entrance into the egg in most animals is incidental; 

 however, this conclusion runs counter to certain con- 

 ceptions, and we shall therefore return to its discussion 

 later. 



With the attachment of the spermatozoon to the 

 egg and its penetration there are always associated 

 certain changes in the cortex of the ovum which vary 

 considerably in their morphological expression in dif- 

 ferent forms. The case of Nereis shows that actual 

 penetration is not necessary for these changes, but the 

 act of penetration is so rapid in most forms that it is 



