No. 2.] THE ECHINODERM SPERMATOZOON. 245 



is approximately spherical. In the Echinoidea (Fig. 3) and in 

 the crinoid studied (Fig. 2) {Antedon rosacea) the spermatozoa 

 are smaller and the head is conical. 



In many species, e.g., HolotJuwia Poll, SticJiopiis regalis, 

 OpJnoglypJia lacetosa, A. glacialis, CJiaetaster longipes, and 

 Cticiimaria planci, in the fresh living spermatozoa there is 

 seen a clearer, more highly refringent apex to the head (Figs. 

 I, 4, and 5); this has been noticed by several investigators, but 

 so far as I am aware no one has previously worked out its 

 origin and its significance. As will be shown below, this body 

 is the sperm centrosome (thus confirming Cuenot's conjec- 

 ture) (4). 



With the exception of the cell membrane enclosing the head 

 of the spermatozoon, and the probable presence of a minute 

 quantity of cytoplasm, the general anatomy has already been 

 often described, and can be readily understood from the 

 figures ; hence we may turn immediately to a consideration of 

 the more detailed history of the various parts. 



The Constituent Elements of the Spermatozoon, Their 



History, 



In treating the subject it will perhaps be advantageous to 

 consider first the anatomy of the mature spermatozoon, the 

 number and nature of its constituent parts ; and then to trace 

 the history of each of these constituent parts by following its 

 successive phases from the spermatogone, through the cell 

 generations following (the spermatocytes and spermatids), to 

 the mature spermatozoon; and then finally to consider the fate 

 of each of these parts in the penetration of the spermatozoon 

 into the egg in the fertilization process. For it is to be borne 

 in mind that the study of spermatogenesis has shown that the 

 spermatozoon is only a modified cell, and that each of its con- 

 stituent parts has previously been a part in a preceding less 

 specialized cell. Each, part, then should be traced from the 

 spermatogone. 



The mature spermatozoon consists of the following parts : 



