OVUM OF A MONKEY IN THE SECOND STAGE. 127 



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 over- 

 lying the decidua serotina. When the blood-vessels of the chorion under the de- 

 cidua reflexa abort, the villi also abort, so that this part of the chorion becomes 

 smooth, and is, therefore, called the chorion lave. Over the serotina the villi con- 

 tinue to grow, hence the corresponding region of the chorion becomes known as the 

 chorion frondosum. The chorion frondosum constitutes the fetal portion, the de- 

 cidua serotina the maternal portion, of the permanent placenta. The maternal blood 

 circulates in the intervillous spaces, which are bounded by fetal ectoderm. The 

 fetal blood circulates in the fetal blood-vessels of the chorionic villi. The circu- 

 latory channels of mother and fetus are always distinct, and no mingling of the 

 maternal and fetal 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, 

 who has also described an almost identical stage of S. pruinosus. 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 diameter. Figure 72 repre- 

 sents 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; on the opposite side it has devel- 

 oped numerous outgrowths, most of which are formed exclusively of the ectoderm, 

 but a few contain an ingrowth of mesoderm in their interior. The ectoderm on 

 the side toward the uterus has two layers, an inner cellular layer with relatively 

 small nuclei, and an outer syncytial or trophodermic layer with larger, nuclei of 

 variable size. The ovum occupies a depression on the surface of the uterus from 

 which the uterine tissues have disappeared, with the result of breaking through the 

 walls of some of the blood-vessels, bl.lac, so that now the maternal blood may es- 

 cape from these vessels into the spaces left between the irregular outgrowths and 

 the embryonic chorion. We must assume that the trophoderm of the embryo has 

 actually dissolved away or digested the tissues of the uterus, thus providing an 

 attachment for the ovum, securing its embedding in the wall of the uterus, and 

 establishing an opportunity for the maternal blood to flow into the intervillous spaces. 

 In later stages of the primates the trophoderm is very much reduced, and therefore 



* Compare Classification of Stages, p. IIQ. 



