SEXUAL RE PROD UC TION. 



135 



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 



Fig. 44. — Different Forms of Conjugation in Plants, a, zoospores; 

 b, mold ; c, d, conjugate alga; ; e,/, desmid. 



Hertwig and Fol have shown that when one sperm has found admit- 

 tance, 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, pathological development is at 

 least often the result. In the lamprey's egg quite a number of sperms 

 find their way into a watchglass-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 fertilization is, as we have just seen, very varied 

 indeed among animals ; what takes place after fertilization is of course cell- 

 division, but that, though referable to certain great types, must necessarilv varv 



