A HUMAN EMBRYO BEFORE THE APPEARANCE OF THE MYOTOMES. 127 



The epithelium lining of the umbilical vesicle varies considerably in different 

 parts. In the axis of the embryo the entoderm consists of flattened cells which 

 form a distinct layer except in the region of the primitive node, head process, and 

 completion plate. Elsewhere in the embryo the entoderm is a definite layer of 

 flattened elements whose nuclei stain less deeply and are possibly a trifle larger than 

 those of the mesoderm just above. In the immediate vicinity of the embryo the 

 walls of the yolk-sac consist of two thin layers of cells, usually closely applied to 

 each other, particularly toward the anterior end, but between the layers occur scat- 

 tered mesoderm cells which are much more numerous in the posterior part of the 

 sac. Farther from the embryo the entoderm cells gradually become thicker, their 

 cell-bodies become more definite, and they take the stain more readily. Over the 

 fundus the lining cells are in general cubical, with well-marked boundaries. Here 

 there is extensive formation of blood-cells and blood-vessels which we shall not dis- 

 cuss at present, except to say that there is no connection between these vessels and 

 those of the body-stalk. In the fundus of the sac there occur two small outpocket- 

 ings of the entoderm into the covering mesoderm (fig. 3). In these diverticula 

 the epithelium is higher and its cells larger than elsewhere. 



THE ALLANTOIS. 



The allantois is given off from the yolk-sac a short distance behind the cloacal 

 membrane. It immediately enters the body-stalk, running at about right angles to 

 the plane of the embryo and, as noted on page 118, is at one point in intimate contact 

 with the amnion. Its length, without reference to its slightly curved course, is 

 about . 65 mm. The lumen is largest just above its origin in a small funnel-shaped 

 depression in the yolk-sac. Its free, slightly coiled extremity has a cavity almost as 

 large as at its origin, while between these the duct and its lumen are narrowest. 

 The average outside diameter is about 0.04 mm. Its walls are composed of low 

 columnar cells containing large, densely-staining nuclei. As it appears in the sec- 

 tions the duct lies in a large space due to the shrinking away of the surrounding 

 tissue. 



THE BODY-STALK. 



The body-stalk is short and distinctly flattened from side to side. Embedded 

 in the loose mesenchyme of which it is composed are the allantois, the posterior part 

 of the amnion, and numerous vessels filled with nucleated blood-cells. No attempt 

 has been made to reconstruct or learn the exact disposition of these channels. In 

 places their walls seem to be deficient and they take on the character of the unlined 

 spaces. The outer covering of the. body-stalk is a rather prominent mesothelium, 

 which is best marked near the embryo and also on what we may call the posterior 

 aspect of the stalk. Toward the chorion this covering stops abruptly, at a varying 

 but short distance from the attachment of the stalk, and there also appear to be 

 deficiencies in this covering, especially on the anterior surface of the stalk. From 

 this mesothelial layer there are a number of more or less definite ingrowths, a few of 

 them forming quite distinct "funnels." Other findings in the body-stalk are the 

 unlined spaces, angiocysts, and angioblast cords described by Bremer (1914). 



