404 A HUMAN EMBRYO OF THE PRESOMITE PERIOD. 



Careful search was made for an amniotic duct and it was found that the 

 amniotic ectoderm, where it lies in contact with the body-stalk, shows at one point 

 an active proliferation and the formation of a wedge which partially penetrates the 

 body-stalk (fig. 14, plate 3). This may, perhaps, represent a tendency toward the 

 formation of an amniotic duct. 



BODY-STALK. 



Loose strands of mesoderm are scattered at irregular intervals throughout the 

 space between the amnion and chorionic membrane, whereas the center of the 

 exocoelom is quite free from them. Within this area of looser mesenchyme is situated 

 the more compactly arranged body-stalk, the form and structure of which can be 

 seen by comparing figure 6, plate 1, and figures 13 and 14, plate 3. Lying at its 

 center is the allantoic stalk, aside from which it consists entirely of a meshwork of 

 mesoderm in which the process of angiogenesis can be seen to be under way. At 

 the margins of the body-stalk the mesoderm is flattened into the mesothelium, 

 which partially separates it from the exocrelom. The amniotic ectoderm bears a 

 very intimate relation to the body-stalk, conforming closely to the shape of its 

 anterior and ventral surface. It is at this point, as has been mentioned before, that 

 the proliferating wedge of ectoderm penetrates into the substance of the body- 

 stalk, representing the amniotic duct. 



YOLK-SAC. 



The yolk-sac is intact and forms a thin-walled, flattened vesicle, rather evenly 

 distended, and measuring 1.5 by 1.4 by 0.9 mm. in its greatest diameters. It con- 

 tains a moderate amount of finely granular coagulum, which is somewhat more 

 abundant in the dorsal portion and around the margins. Over its dorsal pole the 

 wall of the yolk-sac consists of two distinct layers endoderm and visceral meso- 

 blast separated by a narrow cleft, which is bridged here and there by irregular 

 trabeculse. In its ventral two-thirds the visceral mesoblast is intimately adherent 

 to the endoderm, so that in many places it is difficult to make out more than one 

 thin layer. The endoderm is quite uniformly made up of a thin, stretched out, 

 single layer of membrane-like cells. The appearance of the uniformly thin wall of 

 the yolk-sac is interrupted at intervals by small masses constituting the foci of 

 blood-vessel formation. Sketches of these margins are shown on plate 5, and this 

 process will presently be discussed. 



Except at the line along which it fuses with the ectoderm, the endoderm shows 

 very little difference from that in other regions. At the extreme caudal end of the 

 embryonic plate, however, it consists of an area of taller and more cuboidal cells. 

 This area evaginates at the center to penetrate into the substance of the abdominal 

 stalk (fig. 9, plate 2) for a distance of 0.37 mm. as the allantoic duct. This is every- 

 where within the more compact body-stalk. About midway along its course the 

 endoderm of the stalk seems to have broken off and retracted; in the interval, in 

 some of the sections, the empty space previously occupied by it seems to be still 

 present. The allantoic stalk, in some places along its course, shows a distinct 

 lumen, while in other places this can not be recognized. 



