140 



THE HUMAN EMBRYO. 



-Spl. 



Vi, having a very broad connection with the embryo and covered with a network 

 of vessels, in which was a fluid not yet red. A thick body-stalk, Al, can be seen 

 running from the under side of the embryo's tail to the chorion; from the anterior 

 side of the stalk springs the amnion, Am, completely enclosing the embryo. It is 



important to notice that in this, as. in still older embryos, 

 the disposition of the amnion is essentially the same as in 

 the earliest stages; the line of attachment of the amnion 

 is down the sides of the allantois and around the embryo 

 about on a line with the top of the yolk. As regards the 

 embryo, it is drawn slightly canted on to its left side; its 

 back is concave; the head end is thickest; behind and 

 below it can be seen the heart, already a bent tube, 

 shining through; and on the dorsal side, the light-looking 

 oesophagus is distinguishable; in the figure a wedge-shaped 

 shadow intervenes between the straight oesophagus and the 

 bent heart; the heart causes a conspicuous bulging of the 

 body between the head and the yolk-sac; the caudal ex- 

 tremity is thick and rounded and curves upward. Figure 

 85 is a ventral view of the same embryo after most of the 

 yolk-sac has been cut off; its walls, Spl (splanchnopleure), 

 are seen to pass over without any break into those of the 

 intestinal cavity. In the central line the notochord, s, 



can be perceived through the translucent dorsal wall of 



FIG. 85. EMBRYO OF FIGURE ,, . . , . . . a , , , ., , , 



the intestinal cavity; it is flanked on each side by the 



84, SEPARATED FROM THE J ' 



YOLK-SAC AND VIEWED row of square segments. Behind, we see the large body- 



FROM THE UNDER SIDE. stalk, Al, and in front the tubular heart, Ht, with a 



Am, Amnion. Hi, Heart. Spl, decided flexure to the right of the embryo; the anterior 



Splanchnopleure extending j .1 u i -^uj 12 a 



beyond the embryo to form end of the heart makes an opposite bend, separating off a 

 the yolk-sac, s, Noto- limb which becomes the bulbus aorta. The chorion con- 

 chord with a row of s j s t s o f ^ wo layers, one of which forms the uninterrupted 

 inner surface of the chorion, while the outer layer alone 



* 



forms the hollow villi (Figs. 84 and 245); hence, in look- 

 ing at the inside of the chorion, we seen numerous round 



openings which do not penetrate the inner layer. Fortunately, we learn from 

 Kolliker, who had an opportunity in 1861 to examine the chorion, that the outer 

 layer was epithelial, with cells of the same character as in the epithelium of older 

 vascularized villi, and that the inner layer consisted of developing connective tissue, 

 and carried fine blood-vessels. It thus appears that Coste was the first to observe 

 the role of the epithelium in the growth of the villi. 



Human Embryo in the Seventh Stage with One Gill-cleft Showing Externally. 

 No human embryo with only one gill-cleft showing externally is known. 



side. Al, Body-stalk. (After 



Coste.) 



