70 A HUMAN EMBRYO AT THE BEGINNING OF SEGMENTATION, 



the chordal region and the floor of the neural groove. When these regions are 

 figured the defective tissues have been drawn in, but only for the purpose of com- 

 pleting the illustration. In the very roof of the pharynx, where the plane of sections 

 is obliquely tangential, and so again most unfavorable, there are certain appear- 

 ances which might be interpreted as indicating the presence of chordal tissue extend- 

 ing almost as far forward as the buccopharyngeal membrane. Elsewhere in the 

 embryo there is a defect, probably more than a simple tear in the sections, involving 

 the floor of neural groove, and also, but to a much greater extent, the roof of the 

 gut-tract. Along this line, both within the gut cavity and externally in the bottom 

 of the neural tube, is a considerable amount of cell detritus, probably largely 

 chordal in origin. This disintegration of the chorda may be due to certain peculiar 

 properties of the tissue. It might further be construed as evidence of a weak con- 

 nection with the entoderm. Regarding the posterior end of the chorda, nothing 

 whatever can be made out beyond the fusion of ectoderm and entoderm at the 

 primitive node. In general the condition of the chorda is probably much the same 

 as in No. 391, namely, a chordal plate, nowhere entirely free from the entoderm 

 and blending posteriorly with the primitive node. 



The allantois is especially well developed in this case, long and regular through- 

 out its course. Its origin from the yolk-sac has already been noted. It enters the 

 body-stalk, lying near what may be termed the caudal border of the stalk and sur- 

 rounded on the other sides by large anastomosing blood-channels. The lumen is 

 lined by low columnar cells which become more flattened distally, while the caliber 

 gradually increases toward its free, somewhat flattened extremity, which lies very 

 near the tip of the amniotic duct. The length of the allantois is 1.25 mm. and it is 

 everywhere at a considerable distance from the amnion, but its relation to the 

 exoccelom is quite different. Along what we have called the caudal surface of the 

 body-stalk are two prominent longitudinal ridges due to the underlying arterise 

 umbilicales. Underneath the deep groove thus formed, and situated between the 

 vessels, runs the allantois. Here there is a prolonged area of contact between the 

 mesothelium of the body-stalk and the epithelium of the allantois, about 0.3 mm. 

 in extent or about one-fourth the entire length of the duct (fig. 3). The breadth of 

 contact gradually increases as the distal, slightly dilated portion of the allantois is 

 reached, where several epithelial cells are involved. Nowhere is there any inter- 

 ruption or loss of independence in either the epithelium or mesothelium, neither 

 are there any apparent changes in the two cell-layers. What significance, if an}', 

 may attach to this we can not say; at all events it is not of regular occurrence. It 

 may possibly be reminiscent of a larger vascular and functionating allantois, free 



in the exoccelom. 



Immediately dorsal to the origin of the allantois from the yolk-sac, i.e., in the 

 roof of the gut, or yolk-sac, and very close to the caudal limit of the amnion, is the 

 cloacal membrane." It is in a way hardly a membrane, being rather a solid, cylin- 

 drical mass of cells, uniting the ectoderm and entoderm, much as in Wilson's 

 embryo H 3 (1914). 



