144 THE EVOLUTION OF SEX. 



present in appreciable quantity in the water, a recent observer, 

 we are glad to see, claims to have been more fortunate. The 

 spermatozoa, which seem so well to deserve Rolph's epithet of 

 "starved," appear to be powerfully drawn to the well-nourished 

 ovum, and the latter frequently rises to meet the sperm in a 

 small "attractive cone." Often, however, there is an obstacle 

 in the way of entrance in the form of the egg-shell, which may 

 be penetrable only at one spot, well called the micropyle. 

 Dewitz has made the interesting observation, that round the 

 egg-shells of cockroach ova, the sperms move in regular circles 

 of ever varying orbit ; and points out that thus, sooner or later, a 

 sperm must hit upon the entrance. He showed that this was a 

 characteristic motion of these elements on smooth spheres, for 

 round empty egg-shells or on similar vesicles they moved in an 

 equally orderly and systematic fashion. It was till recently 

 believed that more than one sperm might at least enter the 

 ovum, but researches such as those of Hertwig and F'ol have 

 shown that when one sperm has found admittance, the way is 

 usually barred against others. The micropyle may be blocked, 

 or the surrounding membrane may be altered, or in other ways 

 the ovum may exhibit what Whitman calls "self-regulating 

 receptivity," so as to be no longer penetrable. We are safe in 

 concluding, — that the ovum is usually receptive only to one 

 sperm ; that in most cases the entrance of more than one sperm 

 is impossible ; and that where " polyspermy " does occur, patho- 

 logical development is at least often the result. In the lamprey's 

 egg quite a number of sperms find their way into a watch-glass- 

 shaped space at the upper pole of the ovum, but only one gets 

 further, the rest remain imprisoned without further history of 

 any importance. 



What takes place before fertilisation is, as we have just seen, very 

 varied indeed among animals ; what takes place after fertilisation is of 

 course cell-division, but that, though referable to certain great types, must 

 necessarily vary with each species ; what takes place in the act of fertilisa- 

 tion, however, is always essentially the same. The head of the sperma- 

 tozoon becomes the male nucleus (or pro-nucleus) of the fertilised ovum, 

 entering into close association with the female nucleus. The latter, as we 

 have already noted, has had its own history ; it is no longer the original 

 germinal vesicle, nor usually like it in appearance, it is the germinal vesicle 

 minus the cjuantity of nuclear substance given off in forming two polar 

 globules. This female nucleus (or pro-nucleus, as it is generally called) 

 comes into close association with the sperm or male-nucleus ; nor does it 

 remain quite passive in the process, though the greater activity in bringing 

 about the close association is certainly still exhibited by the male. Whit- 



