304 Vctcrhiaty Obstetrics 



FORMATION OF THK DIGESTIVK APPARATUS. 



The alimentary canal of the embryo at first consists of that 

 portion of the blastoderm which is included within the embryo 

 in the process of infolding and finally becomes constricted off 

 from the yolk sac by the gradual folding inward of the embry- 

 onal area at its margins. For a time the alimentary tract con- 

 tinues to communicate with the extra-embryonal portion of the 

 blastoderm, or yolk .sac, by means of the yolk-stalk or vitelline 

 duct. In some species of animals, this tube remains open until a 

 comparatively late period in embryonic life and traces of it may 

 be found in some of them in the umbilical cord at the time of 

 birth, but it does not normally retain its relation with the 

 intestines. 



The intestinal tract, during the early life of the embryo, is 

 divided into three sections, known respectively as the fore-, mid-, 

 and hind-gut ; the first comprising that portion anterior to the 

 communication with the yolk-sac, the second, the middle portion 

 of the area occupied by the yolk-stalk and the third portion, or 

 hind-gut, that which projects from the yolk-stalk posteriorly. 

 In the fore-gut there early appear two dilations, the pharynx 

 and the stomach. The gut is at first clo.sed both anteriorly and 

 posteriori}' and the communications with the exterior beqome 

 established later. The anterior end opens first. 



There appears, in that region of the eml)ryo which is later to 

 constitute the mouth, a depression known as the stomatodeal pit, 

 by which the epiblast sinks inward in the direction of the pharynx 

 until finally the two cavities are separated merely by a thin layer 

 of epiblast and one of hypoblast, the intervening mesoblastic tis- 

 sues having been absorbed. Eventually, the thin membrane 

 gives way and the mouth communicates with the anterior end of 

 the fore-gut, or pharynx. 



The pharynx shows a marked dilation in comparison with the 

 other parts of the digestive tube. In .sagittal section, the inter- 

 nal surface shows the prominent visceral arches, between which 

 extend the visceral pouches, Among these visceral pouches, the 

 hyomandibular and first branchial are the most prominent and 

 have, oji the outer surface, corresponding visceral grooves, which 

 do not normally open upon the exterior, but the hyomandibular 

 groove becomes very thin and ultimately forms the tympanum^ 



