Artliiir AV. Meyer 157 



As all these measurements were made after preservation in alcohol, 

 shrinkage must be borne in mind, although it is of no practical import- 

 ance since estimations of age were not based upon them. 



The vesicles are usually pyriform in shape, somewhat flattened in one 

 diameter, and slightly roughened by protrusion and ridges below which 

 blood islands and blood vessels usually lie. A few specimens are smooth, 

 inflated, translucent sacs without any outward sign of blood islands or 

 blood vessels. Others are collapsed irregularly folded and filled with 

 calcareous-like material. There is never any regularity in the folding 

 of the vesicle, however. Usually the folds were present while the vesicle 

 still lay between amnion and chorion ; while in some cases they were pro- 

 duced during the hardening and imbedding. In several of the inflated 

 vesicles the blood vessels are plainly visible throughout their entire 

 length and can be seen entering the umbilical stalk. 



The umbilical stalk is present in the detached vesicles, as a short 

 (5-15 mm.) stump only. It is thread-like, about .75 mm. in diameter, 

 and never appears twisted. In cross sections the cavity of the vesicle 

 can be traced up to the stalk, and after ending blindly a strand of char- 

 acteristic entodermal cells can be traced for some distance towards the 

 abdominal end; after which the lumen of the stalk reappears at varying 

 intervals. This lumen, which never contains anything but a slight 

 amount of amorphous material, is often completely occluded by the 

 bounding entoderm. 



The stalk itself is composed of three layers in the greater part of its 

 extent. On the exterior there is a thin layer of ccelomic epithelium 

 (mesothelium) which continues indefinitely downward over the vesicle 

 itself. In most vesicles it stops at the upper border, but in three speci- 

 mens it forms a complete outer layer. The entodermal cells which 

 bound the lumen have all the characteristics of those lining the vesicle 

 itself, except for a slight decrease in size. Between these two layers 

 mesoderm is found. Nearer the body of the embryo the latter usually 

 predominates, while it is scarcely represented at all near the upper border 

 of the vesicle. 



Besides these three layers the blood vessels form a conspicuous part 

 of the umbilical stalk. They are not constant in number in various parts 

 of the stalk. Sometimes three arteries and two veins are found, while in 

 other cases one vein and two arteries are present. They can generally 

 be distinguished by the character of their walls. The wall of the vein 

 is formed by a single layer of very flat cells, while that of the arteries 

 usually has an additional outer layer, composed of somewhat flattened 



