HUMAN EMBRYO IN THE EIGHTH STAGE. 



137 



the accompanying figure 72. The medullary tube extends the entire length of 

 the embryo and is the principal component of the head. From the region of the 

 fore-brain has been formed an outgrowth to constitute the optic vesicle, Op. At 

 the side of the hind-brain and on the dorsal side of the pharynx is situated the 

 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, Ht. 

 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 



FIG. 73. HUMAN EMBRYO OF 2.6 MM. LENGTH. (After W. His.} 



oral plate, 0. pi, which consists merely of a thin layer of cells belonging to the 

 ectoderm and entoderm (compare page 100). 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 mus- 

 cular 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 



