48 PROBLEMS OF FERTILIZATION 



relatively small numbers as a' general rule. The study 

 of fertilization is, therefore, usually more difficult techni- 

 cally in such cases; and the process is not readily acces- 

 sible to experimental investigation, when it occurs in 

 the interior of the body. For these reasons such forms 

 have not been used so extensively for study as animals 

 with external fertilization. Nevertheless morphological 

 studies of fertilization, at least, have been made in nearly 

 all classes of the animal kingdom. 



III. THE SPERMATOZOON 



In all animals, excepting the nematodes and Crus- 

 tacea, the spermatozoon is flagellate; it usually exhibits 

 three readily distinguishable parts: head, middle piece, 

 and tail. Within this common morphological form 

 there is the greatest possible diversity of organiza- 

 tion, so that it is probable that the spermatozoon 

 of every species is morphologically distinguishable. 

 Such differences are not usually, however, related in 

 any determinable way to the processes of fertilization 

 themselves. It is indeed probable that certain broad 

 features of difference in organization are adaptive in 

 the sense that they are related to the conditions of 

 fertilization in certain groups; but it seems evident 

 that many of them are results of specific chemical and 

 physical composition in the given environment. It would 

 not be profitable, therefore, to examine their form varia- 

 tions from our point of view, and the series of figures 

 (Fig. i; cf. also Figs. 3, 4, 6, 7) may serve to give an idea 

 of some of the best-known variations in form and size. 



Of the three divisions of the flagellated spermat- 

 ozoon the head is the most massive, containing the dense, 



