142 



THE HUMAN EMBRYO. 



OP, 



Ao 



anlage of the ear, Ot, which at this stage is merely an open invagination of the 

 ectoderm. The region of the mid-brain is marked by the head-bend, so that the 

 axis of the fore-brain is approximately at right angles to the axis of the hind-brain. 

 Another consequence of the head-bend is that the lower process of the head is 

 brought very close to the pericardial chamber enclosing the heart, Hi. Between 

 the head and the pericardial sac is situated the oral invagination or future mouth- 

 cavity, which is still separated from the entodermal 

 canal by the oral plate, O.pl, which consists merely of 

 a thin layer of cells belonging to the ectoderm and en- 

 toderm (compare page 58). The pericardial chamber 

 is large; in the figure only the endothelial portion of 

 the heart, Ht, is represented. Around this endothelial 

 tube is a second and more bulky one from which arises 

 the muscular wall of the heart. The volume of the 

 heart is, therefore, much greater than indicated by the 

 figure, hence the large size of the pericardial chamber. 

 On the dorsal side of the heart, between it and the 

 hind-brain, lies the entodermal canal, which is here the 

 anlage of the pharynx. It has two diverticula or gill- 

 pouches which are not indicated in the figure. On 

 the side toward the mouth the endothelial part is 

 continued beyond the pericardial chamber and gives off 



two vessels on each side, the first and second aortic 

 FIG. 87. RECONSTRUCTION OF THE . 



ANATOMY OF THE EMBRYO arches, which pass around the pharynx and unite again 



SHOWN IN FIGURE 86. upon its dorsal side, and then, as the aortae, Ao, de- 



Op, Optic vesicle, o.pl, Oral plate. scen d along the ventral side of the nervous system, 

 Ht, Endothelial heart. Li. , A . ,, ,. ,. , . , 



, . soon uniting m the median line to form the single 



Liver. Om, Omphalo-mes- 



araic vein. Yk, Yolk-sac, dorsal aorta which runs along nearly to the tail of the 

 All, Aiiantoic diverticuium embryo, where it forks; and its branches, passing one 



formed by the%ntoderm ., Qn each s j de Qf ^ j ntestinal cana} enter the body-Stalk 

 Umbilical vein. Ao, Aorta. ' 



Ot, Otocyst. (After w. His.) an d run to the chorion, where they branch out. Behind 



the pharynx the entodermal canal merges into the cavity 



of the yolk-sac, Yk, and then beyond the yolk-sac extends again into the tail of 

 the embryo, forming an expansion there which is known as the cloaca. From 

 the under side of the cloaca runs out the allantoic diverticuium, All, which extends 

 as a narrow tube of entoderm through the allantoic stalk to the level of the chorion, 

 where it ends blindly. The pericardial chamber on its caudal side is bounded by 

 the septum transversum, in which we find the anlage of the liver, Li, already 

 present, and through which, on either side, the great vein from the yolk-sac, the 

 omphalo-mesaraic or vitelline vein, Om, passes to the heart. Of the veins of the 

 embryo only the umbilical, u.v, is shown in the figure. This vein gathers the 

 vessels from the chorion, passes through the body-stalk, then runs in the somato- 





