38 



HUMAN EMBKYOLOGY. 



The growth at the nodal point not only produces a head fold, but at the same 

 time it forces the cephalic end of the primitive streak caudally over the caudal 

 end of the embryonic area, thus forming a tail fold. 



As the head and tail folds of the embryo are produced by the longitudinal 

 increase of the embryonic area, transverse growth of the area results in the forma- 

 tion of right and left lateral folds (Figs. 37, 39), and as the various folds are 

 formed the embryo rises, like a mushroom, into the interior of the amnion cavity. 



The portion of the entodermal sac which is enclosed within the hollow embryo, 

 formed by the folding of the embryonic area, is the primitive entodermal alimentary 

 canal. The part which remains outside the embryo is the yolk sac, and the passage 

 of communication between the two is the vitello-intestinal duct. 



That portion of the primitive entodermal alimentary canal which lies in the 

 head fold is termed the fore-gut, the part in the tail fold is the hind-gut, and the 

 intermediate portion which is in free communication with the yolk-sac is the 

 mid-gut. 



As the extension of the embryonic area and its folding proceed the margin of 

 the area, which remains relatively stationary, becomes the margin of an orifice, on 



Spinal part of neural tube 



Notochord 

 Fore -gut 



Hind-brairt ^><^"\ \ 



Mid-gut 



Amnion cavity 



Ectoderm of amnion 



Mesoderm of amnion 



Hind-gut 



Mid -brail 



Stomatodaeunf' 



Pericardium (heart not shown 



Rudiment of liver 



i 



i 

 I 

 I 



Umbilical orifice 



Mesoderm of yolk-sac 

 Entoderm 



iculum 



FIG. 50. SCHEMA OF SAGITTAL SECTION OF EMBRYO AFTER THE FOLDING HAS DEFINED BOTH THE 



FORE-GUT AND HIND-GUT AREAS. 



the ventral aspect of the embryo, through which the primitive alimentary canal 

 of the embryo and the intra-embryonic part of the ccelom communicate, 

 respectively, with the yolk sac and the extra-embryonic portion of the coelom. 

 This orifice is the primitive umbilical orifice. 



Not only does the primitive alimentary canal communicate with the yolk 

 sac, and the intra-embryonic with the extra-embryonic ccelom, at the margin of 

 the umbilical orifice, but also the body walls of the embryo, formed by the 

 somatopleure, becomes continuous, at the same margin, with the wall of the amnion. 



The young embryo is connected also with the inner surface of the chorion by 

 a band of tissue which is part of the median portion of the caudal part of the wall 

 of the amnion sac. The mesoderm in this region is thickened, and contains in its 

 interior a diverticulum, allantoic diverticulum, which is primarily derived from the 

 entodermal sac, but is afterwards connected with the hind-gut. This strand con- 

 sists of ectoderm and mesoderm, and it contains not only the allantoic diverticulum 

 but also the blood-vessels passing between the embryo and the chorion. It was 

 called, by His, the body stalk, but the term is not fortunate, for it takes no 

 part in the formation in the body of the embryo. On the other band, its 

 mesodermal and entodermal constituents represent a diverticulum from the wall 

 of the hind-gut, present in many mammals and known as the allantois ; it might 

 with advantage, therefore, be termed the allantoic stalk. 



At first the umbilical orifice is relatively large as contrasted with the total size 



