416 A HUMAN EMBRYO OF THE PRESOMITE PERIOD. 



body-stalk. Just before this point of termination the ectoderm and endoderm 

 unite in the formation of a cloacal membrane. The dorsal wall of the yolk-cavity 

 shows a beginning constriction, which marks off the gut-area. In this respect the 

 v. Spee is slightly more advanced than the Eternod specimen; in it a distinct pouch, 

 corresponding to the fore-gut, is marked off. As regards blood-vessels, it also shows 

 an advance over any of the embryos thus far referred to. 



Throughout groups 2 and 3 we find evidences of blood-vessel formation in the 

 wall of the yolk-sac, in the body-stalk, chorionic membrane and chorionic villi; 

 here, however, for the first time, developing blood-vessels are found in the body of 

 the embryo. Distinct strands, which are to enter into the formation of cardiac 

 endothelium in the v. Spee specimen, are pictured by Evans (1912). Eternod 

 describes in his own specimen a complete blood-circulation. It includes a heart 

 with three or four aortic arches and paired aorta extending back to the body-stalk, 

 where they break up into branches that are distributed to the villi. The blood is 

 returned by two large veins that unite to form a vena umbilicalis impar; this in 

 turn bifurcates into two chorio-placental veins which extend along the junction of 

 the embryo and yolk-sac, to unite in front at the venous end of the heart. From 

 Eternod's description, these vessels are only partially permeable. For the greater 

 part they are in a mesodermal condition which (he says) renders them difficult to 

 trace. It is therefore quite possible that the actual differentiation of the blood- 

 vessels is not so far advanced nor the circulation so complete as one would be led 

 to believe from the author's description. Evans (p. 592) regards it as certain that 

 the structures described by Eternod as aortic arches are not such, but only the 

 components of a vascular plexus. We can, however, safely assume that angio- 

 genesis is to be recognized in the body of the embryo at this time. 



In a summary of the embryos of this group, as compared with those of group 

 2, there is to be included the formation of the medullary groove and medullary 

 folds, the formation of the head-process and its contained canal, the differentiation 

 of the neurenteric canal and the chordal plate, the formation of the cloacal mem- 

 brane, the beginning constriction of the gut-portion from the remainder of the 

 yolk-sac, and finally the evidences of angiogenesis within the body of the embryo. 



PROBABLE AGE OF THE MATEER EMBRYO. 



The sequence of morphological events that mark the developmental period 

 from the earliest-known form up to the appearance of the first somites is now fairly 

 well established and, as can be seen from the preceding review, the embryos therein 

 referred to may be arranged in a consecutive series of clearly defined stages. These 

 may be briefly summarized as follows: 



In the first stage the chorionic villi have not yet appeared and the embryonic 

 rudiment consists of two simple vesicles or of a solid mass of cells in the process of 

 forming these vesicles. In the second stage the villi have made their appearance 

 but are still of a primitive character. In the embryo a mesoblast can be recognized, 

 and is separated into parietal and visceral layers forming an exoccelomic cavity, 

 and the ectoderm can be resolved into the amniotic ectoderm and that forming 



