DEVELOPMENT OF THE OVUM. 813 



in its substance. As the arborescent villi extend, they encroach upon the 

 blood-vessels of the prolongations from the serotina, which latter become 

 much enlarged and finally form the great vascular spaces traversed by the 

 trabeculEe mentioned above. The cells of the serotina form a layer on the 

 walls of the foetal vessels, and the gelatinous, connective-tissue coverings of 

 these vessels have the same origin. Thus the most important parts of the 

 placenta are formed by an interlacement of the villi of the chorion with the 

 altered structures of the mucous membrane of the uterus. 



In the human subject the maternal and foetal portions of the placenta are 

 so closely united that they can not be separated from each other. In partu- 

 rition the curling arteries and the veins on the uterine surface of the placenta 

 are torn off, and the placenta then consists of the parts just described ; the 

 torn ends of the vessels attached to the uterus are closed by the contractions 

 of the surrounding muscular fibres ; and the blood which is discharged is 

 derived mainly from the placenta itself. 



Uses of the Placenta. The placenta is the respiratory, excretory and 

 nutritive organ of the foetus. Its action as a respiratory organ has already 

 been mentioned in connection with the physiology of respiration. It cer- 

 tainly serves as an organ for the elimination of carbon dioxide, and probably 

 also for other products of excretion. It is the only source of materials for 

 the development and nutrition of the foetus. It is thought that the cells 

 derived from the serotina elaborate a fluid called uterine milk, which is ab- 

 sorbed by the foetal tufts. This fluid has been collected 'from between the 

 fostal tufts of the placenta of the cow, and has been found to contain fatty 

 matter, albuminous matters and certain salts, but no sugar or caseine (Gam- 

 gee). It is not certain, however, that such a fluid exists in the human pla- 

 centa ; although " uterine milk " of the ruminants was mentioned distinctly 

 by Haller, and was alluded to by even earlier writers. 



DEVELOPMENT OF THE OVUM. 



The product of generation retains the name of ovum until the form of 

 the body begins to be apparent, when it is called the embryon. At the 

 fourth month, about the time of quickening, it is called the foetus, a name 

 which it retains during the rest of intrauterine life. The membranes are 

 appendages developed for the purposes of protection and nutrition; and. the 

 embryon itself, in the mammalia, is developed from a restricted portion of 

 the layers of cells resulting from the segmentation of the vitellus. 



The formation of the blastodermic cells and the appearance of the groove 

 which is subsequently developed into the neural canal have already been de- 

 scribed. At this portion of the ovum, there is a thickening of the blastoderm, 

 which then presents three layers, the mesoblast, the thickest and most impor- 

 tant, being developed from the opposite surfaces of the epiblast and the hypo- 

 blast. The earliest stages of development have been studied almost exclu- 

 sively in the chick ;. and it is probable that the appearances here observed 

 nearly represent the earlier processes of development in the human subject. 



Development of the Cavities and Layers of the Trunk, in the Chick. As 



53 



