THE PLACENTA. 593 



half an hour, the bitch died from tetanic spasms ; the foetuses 

 operated on were also found dead, while the others were alive 

 and active. The experiments, repeated on other animals with 

 like results, leave no doubt of the rapid and direct transmis- 

 sion of matter from the foetus to the mother, through the blood 

 of the placenta. 



The placenta, therefore, of the human subject is composed 

 of a, fatal part and a maternal part, the term, placenta, prop- 

 erly including all that entanglement of foetal villi and mater- 

 nal sinuses, by means of which the blood of the foetus is en- 

 riched and purified after the fashion necessary for the proper 

 growth and development of those parts which it is destined to 

 nourish. 



The whole of this structure is not, as might be imagined, 

 thrown off immediately after birth. The greater part, indeed, 

 comes away at that time, as the afterbirth, and the separation 

 of this portion takes place by a rending or crushing through 

 of that part at which its cohesion is least strong, namely, where 

 it is most burrowed and undermined by the cavernous spaces 

 before referred to. In this way it is cast off with the foetal 

 membranes and the decidua vera and reflexa, together with a 

 part of the decidua serotina. The remaining portion withers, 

 and disappears by being gradually either absorbed, or thrown 

 off in the uterine discharges or the lochia, which occur at this 

 period. 



A new mucous membrane is of course gradually developed, 

 as the old one, by its peculiar transformation into what is 

 called the decidua, ceases to perform its original functions. 



The umbilical cord, which in the latter part of foetal life is 

 almost solely composed of the two arteries and the single vein 

 which respectively convey foetal blood to and from the pla- 

 centa, contains the remnants of other structures which in the 

 early stages of the development of the embryo were, as already 

 related, of great comparative importance. Thus, in early 

 foetal life, it is composed of the following parts : (1.) Exter- 

 nally, a layer of the amnion, reflected over it from the um- 

 bilicus. (2.) The umbilical vesicle with its duct and apper- 

 taining omphalo-mesenteric bloodvessels. (3.) The remains 

 of the allantois, and continuous with it the urachus. (4.) 

 The umbilical vessels, which as just remarked, ultimately form 

 the greater part of the cord. 



DEVELOPMENT OF ORGANS. 



It remains now to consider in succession the development 

 of the several organs and systems of organs in the further prog- 

 ress of the embryo. 



