DEVELOPMENT OF THE EMBRYON. 911 



Upon the uterine surface of the placenta, is a thin, soft membrane, sometimes called 

 the decidua serotina. This is merely a portion of the mucous membrane of the uterus 

 situated next the muscular walls, the greater part of it not being thrown off with the pla- 

 centa. It is composed of amorphous matter, numerous granulations, and colossal cells with 

 enlarged and multiple nuclei. If we scrape the uterine surface of a fresh placenta, these 

 cells appear, upon microscopical observation, very much like the so-called cancer-cells. 

 There has been and is now considerable difference of opinion with regard to the formation 

 of the decidua serotina. Some writers, who do not admit that the placenta has any true 

 maternal portion, regard it as the portion of decidua imprisoned between the chorion 

 and the muscular walls of the uterus ; but, if we adopt the view that the placenta ia 

 formed in part of the uterine mucous membrane, we must regard the serotina, so called, 

 as simply the deeper portion of this membrane. 



Blood-vessels of the Placenta. The two arteries of the umbilical cord branch upon 

 the foetal surface of the placenta beneath the amnion and finally penetrate the substance 

 of the organ. The branches of the veins, which are about sixteen in number, converge 

 toward the cord and unite to form the umbilical vein. Upon the uterine surface of the 

 placenta, are numerous oblique openings of the veins which return the maternal blood to 

 the uterine sinuses. There are also numerous small spiral arteries, which pass into the 

 substance of the organ to supply blood to the maternal portion. These are the " curling 

 arteries," described by John Hunter. 



If we inject the umbilical arteries, the fluid is returned by the umbilical vein, having 

 passed through the vascular tufts of the fcetal portion of the placenta. According to 

 Farre, the small arteries and the veins of the villi at first communicate through a true 

 capillary plexus ; but, toward the end of pregnancy, the capillaries disappear, leaving 

 loops of vessels, " simple, compound, wavy, or much contorted, and in parts varicose." 



According to the recent researches of Winkler, there are three kinds of foetal villi : 

 1. Those which terminate just beneath the chorion, without penetrating the vascular 

 lacunae. 2. Longer villi, which hang free in the lacunae. 3. Long, branching villi, which 

 penetrate more deeply into the placenta, some extending as far as its uterine surface. 



The formation of the great vascular lakes of the maternal portion of the placenta has 

 already been described. These, according to Winkler, present numerous trabeculse, 

 which extend from the uterine to the foetal surface ; and, between these trabeculae, are 

 numerous exceedingly delicate transverse and oblique secondary trabecular processes. 

 The chorionic villi contain blood-vessels, which we have already described, surrounded 

 by a gelatinous, connective-tissue structure {Schlewigewebe), and are generally covered 

 with a layer of nucleated cells of pavement-epithelium. 



In parturition, the curling arteries and the veins on the uterine surface of the pla- 

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

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

 surrounding muscular fibres ; and the blood which is discharged is mainly derived from 

 the placenta itself. Thus the very contractions which expel the contents of the uterus 

 close the vessels and prevent loss of blood by the mother. 



Development of the Embryon. 



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 intra-uterine 

 life. The membranes which we have described are appendages developed for the pur- 

 poses 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. 



We have already described the formation of the blastodermic cells and the appearance 



