TUBIFEX. 353 



cytoplasm being much reduced in quantity. Gradually, 

 however, the spermatids become oval in shape. The 

 nucleus of the spermatid then elongates considerably to 

 form the filiform head of the spermatozoon so 

 characteristic of the group (PI. V, fig. 34). The middle 

 piece of the spermatozoon is not very conspicuous, but 

 appears as a direct continuation of the head. The distal 

 extremity of the spermatid now becomes much drawn 

 out, forming a long tail which is, in the normal sperma- 

 tozoon, considerably longer and thinner than the head. 

 It stains but faintly with nuclear stains, as it is composed 

 of cytoplasm only, derived in all probability from the 

 delicate layer of protoplasm surrounding the nucleus of 

 the spermatid. At first, the fully formed spermatozoa 

 are arranged very regularly around the blastophore, as 

 regularly, in fact, as were the spermatids from which they 

 were derived, each one with the anterior end of the head 

 buried in the substance of the blastophore. The tails of 

 the spermatozoa are at first quite straight, and while still 

 in this condition become motile (PI. V, fig. 35). When the 

 spermatozoon is quite matured and ready to leave the 

 blastophore, the tail usually exhibits a simple coil at the 

 free end. The spermatozoa now become much less 

 regularly arranged on the blastophore and by degrees 

 they separate completely from it. When they are set 

 free they remain for a time in the cavities of the sperm 

 sac, but finally they find their way to the ciliated funnel 

 of the vas deferens, around which, in the mature worm, 

 they congregate in great numbers. 



In most of the individuals I have examined, I have 

 found developing in the sperm sac, side by side with the 

 normal spermatozoa, other structures which resemble 

 them to a certain extent, but which can easily be 

 distinguished from them. At first I was inclined to look' 



