1246 



THE ORGANS OF DIGESTION 



cavity left outside the embryo, the extra-embryonic coelom, envelops the vitelline 

 or yolk sac. 



A m ni o 



-AUantois 

 Hind-gut 



FIG. 962. Diagram of a longitudinal section of a mammalian embryo. Very early. (After Quain.) 



The relatively slow growth of the margin of the embryonic area and the relatively 

 rapid growth of the axial parts soon come to form a ring of constriction between 

 the embryo and the yolk sac, and a part of the latter that is, a part of its splanch- 



nopleural wall and part of the 

 cavity becomes enclosed within 

 the embryo to form the primitive 

 alimentary canal (Fig. 962). At 

 the same time a part of the ccelom 

 becomes enclosed within the em- 

 bryo by the relative approximation 

 of the lateral, cephalic, and caudal 

 folds, and the embryonic coelom 

 cavity forms the rudiment of the 

 pleural, pericardial, and peritoneal 

 cavities. The abdominal cavity is 

 separated from the other cavities 

 by the septum transversum, the 

 proton of the Diaphragm. The 

 embryo grows more rapidly in 

 length than in width, and its 

 cephalic and caudal extremities 

 are bent ventrad; the forward 

 growth of the head tilts the areas 



iker," after Bischoff.") situated cephalad so that the posi- 



tions of the buccopharyngeal and 



pericardial areas become reversed; the same inversion takes place with regard to 

 the cloacal membrane. 



FIG. 963. Early form of the alimentary canal. In A a 

 front view, and in B an antero-posterior section are repre- 

 sented, a. Four pharyngeal or visceral plates. 6. The 

 pharynx, c, c. The commencing lungs, d. The stomach, 

 f, /. The diverticula connected with the formation of the 

 liver, g. The yolk sac, into which the middle intestinal 

 groove opens, h. The posterior part of the intestine (From 

 Kollil "-" " 



