232 REPRODUCTION 



113 and 115 ). When liberated in the proper medium, the sperma- 

 tozoon swims actively until it dies or meets an ovum in fertiliza- 

 tion. The structural and functional resemblances between sperma- 

 tozoa and the more speciahzed forms of microgametes in protozoa 

 are obvious (cf. Figs. 115 A and 108, 9b). 



Fertilization. — The union of ovum with spermatozoon, which 

 constitutes the " fertilization " of the egg, may be described in 

 general terms, since it occurs in much the same manner in all 

 animals. The process has been most thoroughly studied in eggs 

 that are fertilized in water after discharge from the parent, as with 

 many marine animals; but, so far as the observations go, there 

 seems to be little difference between such cases and the fertilization 

 of a grasshopper's or a mammal's egg in the fluid of an internal 

 cavity of the female reproductive system. The present account 

 has reference to the process as it occurs in the sea-urchin, Tox- 

 opneustes (Fig. 116), in which the maturation of the ovum is 

 delayed until the time of fertilization. After being discharged 

 into the water, the egg remains suspended or slowly sinks to 

 the bottom while the sperm swim about actively by means of 

 the lashing of their flagella. When such a spermatozoon comes 

 in contact with the surface of an ovum and penetrates the egg 

 membrane, the movements of its flagellum cease and the surface 

 of the ovum forms the so-called entrance cone by which the head 

 and middle-piece of the spermatozoon become enclosed. From this 

 time on, the spermatozoon appears as though it were inactive and 

 were being drawn into the ovum by the action of the latter's cyto- 

 plasm rather than by any movements of its own. In many ani- 

 mals the flagellum remains on the outside, and even where it nor- 

 mally enters, as in the birds, it takes no further part in the devel- 

 opment. As the entrance cone develops, the egg produces on its 

 external surface what is known as the fertilization memhrane. 

 This was formerly beheved to be the mechanism by which the 

 entrance of other spermatozoa was prevented, but is now regarded 

 as the incidental result of changes produced in the ovum by the 

 presence of the spermatozoon. These changes in some way render 

 the surface of the ovum impervious to other spermatozoa within 

 a few minutes, or even a few seconds, after the entrance cone has 

 made its appearance. In this manner polyspermy, or the presence 

 of more than one spermatozoon in the ovum, is prevented unless 

 the sperm are so abundant that several reach the surface of the 



