REPRODUCTION AND SEX 477 



the cilia beat strongly towards the lower end of the duct, how do 

 the microscopically minute spermatozoa make their way against the 

 stream ? 



Various suggestions have been made. Thus it is said that the 

 constitution of the sperms is such that they go against a current, 

 just like elvers going up the river. In other words, their tropism 

 is to move against a current; and of this there is some experimental 

 evidence in cases where the current is not too strong. But here it 

 is interesting to notice the remark of Prof. G. H. Parker of Harvard, 



Fig. 64. 



Diagram of Typical Spermatozoon. A, peak or acrosome ; H, head, containing 

 a minimal amount of cytoplasm (CY) and a number (4) of chromosomes ; 

 MP, the middle piece, including the centrosome (C) ; T, the tail, with axial 

 filament (AF). 



whose investigations are always very suggestive: "No one can have 

 examined the ciliary current in a living oviduct without having 

 been impressed by its vigour. In fact, so strong is it that I have 

 never been able to convince myself that spermatozoa could make 

 headway against it, and yet these cells must, by one means or 

 another, reach the proximal end of the oviduct in the neighbourhood 

 of which the egg is fertilised." 



It has also been suggested that the seminal fluid may bring 

 about a reversal of the ciliary current, so that it helps the sper- 

 matozoa up the oviduct. Such a reversal is known in other cases, 

 but there is no experimental proof that it occurs in the oviduct. 



