390 HUMAN PHYSIOLOGY 



rendered firm by engorgement with blood it is adapted to 

 be thrust into the vagina and so to convey the seminal 

 fluid to the very portal of the uterus. This constitutes 

 the act of coitus. Excitement is obviously necessary in 

 the male; it is not necessary, though normal, in the 

 female. 



The ovum is a microscopic sphere answering well to our 

 idea of a typical cell. It has no inherent power to move, 

 but is transmitted by currents of ciliary or other origin. 

 The spermatozoa, on the contrary, are highly active swim- 

 ming elements. Each one has an oval "head-piece," a 

 minute "middle-piece," and a lashing tail or flagellum by 

 which it is propelled. Its progress suggests that of a 

 tadpole. When a host of these motile cells are set free 

 high up in the vagina the chances favor the entrance of 

 some of them into the uterus and a smaller number may 

 even reach the Fallopian tubes. In one of these passages 

 the fertilization of an ovum may then be accomplished. 



The seemingly prodigal production of spermatozoa of 

 course increases the chance that this will occur. Only one 

 sperm can take part in the actual impregnation. Among 

 lower forms of life, such as star-fishes, we can watch the 

 entrance of the sperm into the ovum. As the boundary is 

 passed the tail, now superfluous, may be detached and left 

 outside. A prompt chemical alteration in the egg must be 

 inferred to take place, for it seems at once to be protected 

 against the intrusion of other spermatozoa, which are 

 likely enough to blunder against it. We have every reason 

 to believe that the facts correspond for the higher types 

 which do not admit of such direct observation. In many 

 cases thick envelopes surround the ova and superfluous 

 sperm-cells often stop in these external zones. 



We have said that a cell usually contains a nucleus as a 

 conspicuous feature of its organization. The infertile 

 ovum has such a nucleus surrounded by a relatively large 

 volume of cytoplasm. The sperm also has a nucleus we 

 may almost say that it is a nucleus, for it has little other 

 substance. The central fact in fertilization appears to be 



