164 A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMITES. 



approach each other, so that the dorsal surface of the body-stalk (dorsal with 

 reference to the embryo) lies within the amnionic cavity. Some little distance 

 beyond the tip of the tail of the embryo, at the distal end of the body-stalk, the two 

 lines come together and complete the continuous line of reflection. The entire 

 anterior half, a small portion of the caudal end, and the dorsum of intervening por- 

 tions of the embryo and dorsum of the body-stalk lie within the amnionic cavity. 



HISTOLOG1CAL STRUCTURE. 



Structurally the inner layer of the amnion is composed of a single layer of 

 cells. These are for the most part cubical or rounded in shape and he close 

 together, but in places where they are further spread apart they are greatly 

 flattened. The flattening affects only the protoplasm of the cells, the nuclei 

 remaining rounded. The cells of the outer layer are also rounded and contain 

 rounded nuclei. They are so placed that their inner surfaces, i. e., the surface 

 which is directed toward the embryo, are all joined together along the mesen- 

 chyma, while the remaining portions of the cells are free from one another. The 

 cells therefore project outward away from the amnion, each forming a rounded 

 protuberance. In many places these cells are columnar or pear-shaped, the 

 nuclei being distally placed. Between the two layers of epithelium is found a 

 thin layer of mesenchymal tissue, which is exceedingly poor in cells. 



In the region of the yolk-stalk the amnion is somewhat thicker along its line 

 of reflection. It is greatly pitted by branching depressions from the exocoelomic 

 cavity lined with the typical mesothelium, which, when cut in cross-sections, give 

 the appearance of blood-vessels. Some of these depressions are shown in plate 7, 

 figures 1 and 3. Similar pit-like depressions have also been found over the greater 

 part of the amnion and all along its line of reflection, on the left side particularly, 

 and on the body-stalk, where the amnion is reflected from it (plate 7, fig. 2) . 



Although the coelomic depressions when seen in cross-section resemble blood- 

 vessels at first glance, closer examination shows that they are easily distinguishable 

 from them. The shapes of the cells of the endothelial tubes are more flattened 

 than those of the mesothelial ; the endothelial nuclei are elongated, while those of the 

 mesothelium are rounded; and in the blood-vessels hemogloblin-bearing blood-cells 

 are usually found, while in the coelomic depressions they are always absent. 



I shall here mention the recent work of Bremer 3 regarding the formation 

 of the earliest blood-vessels in man. This author finds funnel-shaped growths of 

 the surface mesothelium in the yolk-sac and in the body-stalk, which he believes 

 give rise to a network of blood-vessels. That the pit-like depressions which I 

 have just described resemble in a way the funnel-shaped ingrowths of Bremer's 

 description is apparent. Judging from Bremer's figures, the pits in my specimen 

 are undoubtedly larger and more numerous, but it must be noted that my embryo 

 is somewhat older than any of those which Bremer describes. I am, however, 

 unable to make out any evidences of blood-vessel formation taking place from 

 these depressions. No definite connections are apparent anywhere between the 

 depressions and the blood-vessels. 



