HUMAN EMBRYO IN THE EIGHTH STAGE. 135 



ous bulging of the body between the head and the yolk-sac ; the caudal extremity 

 is thick and rounded and curves upward. Figure 70 is a ventral view of the same 

 embryo after most of the yolk-sac has been cut off; its walls, Spl (splanchno- 

 pleure), are seen to pass over without any break into those of the intestinal cavity. 

 In the central line the chorda dorsalis, s, can be perceived through the translucent 

 dorsal wall of the intestinal cavity ; it is flanked on each side by the row of square 

 segments. Behind, we see the large body- 

 stalk, Al, and in front the tubular heart, ^ ^iil~%^^ < '^^ v -'i%v 

 Ht, with a decided flexure to the right of ^^m^ff^y^^^'^ I / p 

 the embryo; the anterior end of the heart c r^ . f 



makes an opposite bend, separating off a 7 



limb which becomes the bulbus aorta. The $\ 

 chorion consists of two membranes, one 

 of which forms the uninterrupted inner 

 surface of the chorion, while the outer 

 membrane alone forms the hollow villi 

 (Figs. 69 and 201); hence, in looking at 

 the inside of the chorion, we see numerous FlG 7 ,._ HU MAN EMBRYO, 2.15 MM. LONG. 

 round openings which do not penetrate the (After w. His.) 



inner membrane. Fortunately we learn 



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

 outer membrane was epithelial, with cells of the same character as in the epithe- 

 lium 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. 



Human Embryo in the Eighth Stage with Two Gill Clefts Showing Externally. 



Several embryos in this stage have been described and some of them 

 studied anatomically. Those which are best preserved and which we have 

 best reason to think are normal present a very singular appearance, owing to 



merits were visible externally. The shape of the head, the size and curvature of the heart, the form of the tail, 

 and the concavity of the dorsal outline in the segmented region of the embryo all indicate an extremely close 

 resemblance to Kollmann's embryo. As Coste's figures were all made from fresh specimens freehand, we shall 

 probably commit no error if we assume that the magnification was not correctly given. By making this assump- 

 tion I think the difficulties as to placing Coste's embryo vanish. 



Coste's private collection was said to be at the College of France, but upon search this specjmen could not 

 be found, so that attempts to ascertain its actual length were without result. 



