CHAPTER II. 



PREGNANCY AND BIRTH. 



SEC. 1. THE PLACENTA. 



693. 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 degenera- 

 tive 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 (whicli 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, developes into 

 an embryo. 



694. 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 new growth does not give way to 

 haemorrhage and immediate decay; it remains, and nuy U- <1U- 



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