158 The Structure of Human Umbilical Vesicle 



entodermal cells. This difference in structure, which is evident with 

 the low power of the microscope, is found to disappear soon after the 

 upper border of the vesicle is reached. In the structure of the walls 

 of the blood vessels of the yolk sac itself there is never any difference as 

 far as I am able to ascertain. The position of the vessels in both stalk 

 and vesicle is usually well out towards the periphery, and in some cases 

 only the ccelomic epithelium covers them. 



For the microscopic structure of the youngest umbilical vesicles refer- 

 ence to the literature is necessary. Peters, in his monograph, gives the 

 size of both embryo and vesicle as 0.19 mm. Unfortunately, the preser- 

 vation of the umbilical vesicle of Peter's ovum was not such as to prompt 

 a detailed description of it. We are told, however, that it is composed 

 of entoderm and mesoderm, and in the accompanying plate (Peters, Taf. 

 Ill, Fig. 33) some contents containing globules and cells are represented. 

 In this plate the lower half of the vesicle shows no clear demarcation 

 between mesoderm and entoderm; while in the upper half a fairly clear 

 line of division between the two is indicated. The character of the 

 mesodermal and entodermal cells is not given in the monograph, except 

 that the latter are spoken of as " unscheinbaren Entodermzellen." Blood 

 islands and blood vessels are not represented. 



In an embryo of 0.37 mm. described by Graf Spee a marked advance 

 in the structure of the umbilical vesicle exists. In this case the ento- 

 derm, which is one-layered, is composed of cubical cells, while the meso- 

 derm is made up of irregular masses of cells with protrusions on the 

 distal half of the vesicle, below which blood islands are found between 

 entoderm and mesoderm. The' latter is thus pushed out while the 

 entoderm in these places is said to be more wavy, its cells of greater 

 variety and stained more intensely. 



The distal part of Graf Spee's embryo Gle (an embryo 1.54 mm. long) 

 is said to be full of gaps — " ausserst liickenreich." Some of these gaps 

 have an epithelial lining of flat cells of the nature of embryonic endo- 

 thelium. Blood Anlagen are found in the wall of the vesicle only. In 

 the proximal third of the latter the entoderm and mesoderm are thin 

 and membranous, while in the distal two-thirds they are of varying 

 thickness. The protrusions on the surface are said to be due to collec- 

 tions of cells between . entoderm and mesoderm. It is of interest to 

 note in this connection that Keibel states that the umbilical vesicle of an 

 embryo 1.0 mm. long, described by him, is in every particular like that 

 of embryo Gle of Graf Spee. 



In the later article of Graf Spee, already referred to, he says that in 

 embryos of three or four weeks the entoderm forms true glandular struc- 



