68 A HUMAN EMBRYO AT THE BEGINNING OF SEGMENTATION, 



ENTODERM AND DERIVATIVES. 

 The gut-tract (fig. 3) is represented at this stage by a well-defined fore-gut and 

 what is combined mid-gut and hind-gut, There is as yet no real hind-gut, since the 

 cloacal membrane is in the roof, while just behind this the allantois comes off from 

 a rather deep, laterally compressed, funnel-shaped outpocketing of the yolk-sac, 

 the roof of which is formed by the entoderm in the region of the cloacal membrane 

 and primitive streak. 



The fore-gut, about 0.5 mm. in length, makes a very decided angle with the 

 future mid-gut, due to the erection of the whole anterior end of the embryonic 

 body. It is at this time quite large and capacious, determining, together wtih the 

 neural folds and heart, the configuration of this part of the embryo. Dorsally, in 

 the mid-line, chordal region, it is in close relation throughout with the thin floor 

 plate of the neural groove. In general, the dorso-ventral diameter is distinctly 

 less than the transverse; its anterior half is definitely enlarged, especially in its 

 transverse measurements, the future oral and pharyngeal regions. The shape of 

 the cavity of the fore-gut varies considerably in its anterior and posterior portions. 

 In the latter, from just behind the first pocket and thyroid to its communication 

 with the yolk-sac, it may best be described as triangular or even T-shaped on sec- 

 tion. Internally the dorsal wall is concave in the median line and then passes out 

 to the roof in the rounded lateral angles. Externally, in the roof of these lateral 

 recesses, on either side of the median ridge, are two rather faint, broad furrows 

 which may be termed aortic and which are lost anteriorly on the roof of the pharynx. 

 The two ventro-lateral walls appear as if pushed in toward the center, with the 

 result that there is present a deep median trough in the floor and two lateral some- 

 what shallower grooves on either side, but close to the roof of the gut, 



Externally, the fore-gut shows a prominent ventral keel corresponding to the 

 internal median groove and two laterally projecting ridges which fade out behind 

 on the yolk-sac. On either side of this keel the entodermal walls are concave, the 

 dorsal wall is in general convex, the opposite holding for the interior. This ventral 

 keel is in intimate relation with the developing heart, projecting through as it were 

 between the two layers of the wide mesocardium until it lies quite close to the endo- 

 thelial tube of the heart, The two dorsally placed, lateral grooves or recesses in the 

 gut-wall lie behind the attachment of the mesocardium and behind the dorsal wall 

 of the pericardial ccelom, projecting toward, but at some distance from, the begin- 

 ning pleuro-peritoneal passages. Traced forward, the ventro-lateral walls of the 

 gut open out as the space demanded by the developing heart gradually decreases. 

 The dorso-lateral grooves likewise fade out as the result of these changes in the 

 lateral walls. Just where these grooves merge into the enlarged anterior part of 

 the fore-gut we may locate the earliest trace of the first entodermic pocket. On 

 the right side there appears a small furrow, which extends for a short distance 

 ventrally and very slightly posteriorly across the lateral wall, where it comes into 

 relation with a second groove, which, if continued, would cut the median line just 

 in front of the thyroid (fig. 4) . 



