734 GENERATION AND DEVELOPMENT. 



comparison with, the size of the spermatic artery, the 

 blood must move very slowly. 



The seminal fluid secreted by the testicle is one of those 

 secretions in which a process of development is continued 

 after its formation by the secreting cells, and its discharge 

 from them into the tubes. The principal part of this 

 development consists in the formation of the peculiar 

 bodies named seminal filaments, spermatozoa or spermatozoids 

 (fig. 213), the complete development of which, in their full 

 proportion of number, is not achieved till the semen has 

 reached, or has for some time lain in, the vesiculse semi- 

 nales. Earlier, after its first secretion, the semen contains 

 none of these bodies, but granules and round corpuscles 

 (seminal corpuscles), like large nuclei, enclosed within 

 parent-cells (fig. 213). Within reach of these corpuscles, 

 or nuclei, a seminal filament is developed, by a similar 

 process in nearly all animals. Each corpuscle, or nucleus, 

 is filled with granular matter ; this is gradually converted 

 into a spermatozoid, which is at first coiled up, and in 

 contact with, the inner surface of the wall of the corpuscle 

 (fig. 213, C, i). 



Thus developed, the human seminal filaments consist of 

 a long, slender, tapering portion, called the body or tail, 

 to distinguish it from the head, an oval or pyriform por- 

 tion of larger diameter, flattened, and sometimes pointed. 

 They are from 5 ^ th to -g-^th of an inch in length, the 

 length of the head alone being from -o'oirth to -^-^th of 

 an inch, and its width about half as much. They present 

 no trace of structure, or dissimilar organs ; a dark spot 

 often observed in the head, is probably due to its being 

 concave, like a blood corpuscle. They move about in the 

 fluid like so many minute corpuscles, with each a ciliary 

 process, lashing their tails, and propelling their heads 

 forwards in various lines. Their movement, which is pro- 

 bably essentially, as well as apparently, similar to that of 

 ciliary processes, appears nearly independent of external 





