OVUM OF A MONKEY IN THE SECOND STAGE. 121 



become very well developed, attaining a height of from 5 to 15 mm. They are 

 irregularly disposed, but subdivide the placenta of later stages into the so-called 

 cotyledons (compare page 337). 



The body-stalk becomes converted into the umbilical cord. This cord runs 

 from the body of the embryo to the chorion (Figs. 55 and 72). It is always con- 

 nected with that portion of the chorion which is adjacent to the decidua serotina. 

 It carries the arteries and veins from the body of the embryo to the chorion. 

 From the end of the umbilical cord the blood-vessels branch out over the chorion 

 and into the chorionic villi. Thus the chorionic circulation of the embryo centers 

 about the chorionic end of thefumbilical cord, and as this end is in the part of the 

 chorion overlying the decidua serotina, we have here established from the very 

 start an important factor in the further differentiation. From what has been said 

 it is evident that the portion of the chorion underlying the decidua reflexa is more 

 remote from the center of the embryonic circulation. In the same way we find 

 that the decidua reflexa is remote from the blood-supply in the uterus, and, as a 

 matter of fact, we may observe that during the second month of pregnancy the 

 blood-vessels, both in the decidua reflexa and in the portion of the chorion near 

 it, begin to disappear and ultimately are completely atrophied. After this 

 atrophy has been accomplished the circulation of the chorion is restricted to that 

 portion overlying the decidua serotina. When the blood-vessels of the chorion 

 under the decidua reflexa abort, the villi of that region also abort, so that that 

 part of the chorion becomes smooth, and is, therefore, called the chorion Iceve. 

 Over the serotina the villi continue to grow, hence that region of the chorion 

 becomes known as the chorion frondosum. The chorion frondosum constitutes 

 the foetal portion, the decidua serotina the maternal portion, of the permanent 

 placenta. The maternal blood circulates in the intervillous spaces, which are 

 bounded by foetal ectoderm. The foetal blood circulates in the foetal blood- 

 vessels of the chorionic villi. The circulatory channels of mother and foetus are 

 always distinct, and no mingling of the maternal and foetal blood is possible 

 under normal conditions. 



Ovum of a Monkey in the Second Stage. 



This embryo was obtained from a Semnopithecus nasicus in Borneo by 

 Selenka. The ovum represents the earliest stage of any primate yet known. 

 It rested against the wall of the uterus and was uncovered, there being no decidua 

 reflexa developed in monkeys. It measured about 2 mm. in its greatest diam- 

 eter. Figure 57 represents a section through the ovum and adjacent tissues of 

 the uterus. The chorionic vesicle is very large, but the embryo, Sh, and yolk- 

 sac, Yk, are relatively very small. The chorion on one side is quite smooth; 



