EMBRYO OF 9 MM. STUDIED IN SECTIONS. 253 



of the umbilicus. It consists of a very small tube of entoderm, Ent, with only a 

 small internal cavity (compare Fig. 186, Reel.}. The thickness of the intestinal 

 wall is due chiefly to the considerable development of the mesenchyma. The ex- 

 ternal covering of the intestine is a layer of mesothelium which becomes the peri- 

 toneal epithelium of the adult. In the tissue of the organ we distinguish the 

 narrow vitelline artery, Art.v. The umbilical opening is very wide and is bounded 

 both above and below by a prolongation, Um.w', Um.w", of the somatopleure 

 of the embryo. The wall on the upper side is much thicker than on the lower. 

 The umbilical opening is partly occupied by the duodenum. Appended to 

 the inferior wall of the umbilicus is the allantois, All, which arises from the 

 enlarged caudal end (cloaca), Clo, of the intestine. It passes out first inward, 

 then makes an acute but rounded angle, and extends outward through the um- 

 bilical opening. It may, therefore, be said to consist of two limbs, one within 

 the body of the embryo joining the cloaca, and the other passing out through 

 the umbilical opening. The limb arising from the cloaca is completely united 

 with the body-wall, and is, of course, upon the side toward the ccelom covered 

 in by mesothelium. The lining of the allantoic cavity is an epithelium, and is a 

 portion of the entoderm. Along the second limb of the allantois the mesothelium 

 on the side toward the cavity of the umbilicus forms a series of clumsy projections, 

 Vil, the mesothelial ^illi of the allantois. They are smallest toward the embryo 

 and increase in size distally. With higher power one can see that the mesothelium 

 of the villi is very thin and the mesenchyma in their interior of quite loose texture. 

 In later stages the mesothelium grows, the mesenchyma in large part disappears, 

 and the villi then seem hardly more than small bags of mesothelium with but 

 little contents, save some coagulum. They continue to enlarge until the embryo 

 is 17 or 1 8 mm. long, after which they begin to abort. In these older stages the 

 villi extend far into the abdomen and are packed in between the abdominal 

 viscera, presenting curious appearances in section. As the tail of the embryo is 

 bent to one side, it offers us a section of a portion of the spinal cord, Sp.c, and 

 at its tip a glimpse of three primitive somites, Seg. 



Frontal Section through the Mid-brain and Fore-brain (Fig. 185). Comparison 

 with figure 165 (pig, 7.5 mm.) will make it clear that in a frontal series 

 obtain a few sections of the head which include only mid-brain and fore-brain a>id 

 show no other special cephalic structures. The mid-brain, M.B, is somewhat 

 rounded in form and passes over into the fore-brain, which is quite long and 

 which already shows traces of its subdivision into two parts, the diencephalon, 

 Dien, which lies nearest to the mid-brain, and the prosencephalon, Pros, which 

 constitutes the terminal portion of the brain and which produces the lateral expan- 

 sions which are to form the cerebral hemispheres. The expanding prosencephalon 

 is separated by a constriction from the diencephalon, which in its turn is similarly 

 separated from the mid-brain. The diencephalon and prosencephalon together rep- 

 resent the fore-brain. They are subdivisions of the primary first cerebral vesicle. 



