CHAPTER II. 



PREGNANCY AND BIRTH. 

 SEC. 1. THE PLACENTA. 



949. THE spermatozoa travelling up the female passages 

 come in contact with the ovum. Making their way through the 

 cells of the discus, which by this time are undergoing degenerative 

 changes, and piercing the zona pellucida, they enter the vitellus ; 

 it is stated that as a rule one spermatozoon only actually reaches 

 the vitellus. Here the tail, which by its vibratile activity has 

 thus brought the spermatozoon to its destination, ceases to move 

 and soon disappears; but the head (and we have seen that the 

 head is a prepared and, so to speak, purified nucleus, a male 

 pronucleus) unites with the pronucleus of the ovum to form the 

 nucleus of the now impregnated ovum. 



As the result of this action of the spermatozoon on the ovum, 

 the latter, instead of dying as when impregnation fails, awakes to 

 new nutritive activity. It undergoes segmentation, the one cell 

 becomes by cell-division a mass of cells, which, passing through 

 a series of remarkable morphological changes, into the details of 

 which we cannot enter here, develops into an embryo. 



950. No sooner, however, have these changes begun in the 

 ovum than correlative changes, brought about probably by reflex 

 action, but at present most obscure in their causation, take place 

 in the uterus. The mucous membrane of this organ, whether the 

 coitus, which was the cause of the impregnation, took place at 

 a menstrual period or at some time in the interval, undergoes 

 changes which though more intense are at first not unlike those of 

 menstruation; it becomes congested, and a rapid growth takes 

 place, characterized by a proliferation of the epithelial and other 

 tissues. Unlike what takes place in menstruation, however, this 



