Arthur W. Meyer 163 



the entire wall of the vesicle is composed of entoderm (Fig. 5), for in 

 young specimens the entoderm is composed of a single well-defined layer 

 of cells (Fig. 1). 



In embryos of the seventh to tenth week the entoderm and sometimes 

 the tubules can be found in various degrees of degeneration. This is 

 true of Nos. 145, 176 and IS-t, embryos of 33, 38 and 50 mm. long, 

 respectively. As the mesoderm is generally increased in thickness in 

 these specimens it seems as though the degeneration of the entoderm is 

 accompanied by a proliferation of the mesoderm. The latter at this 

 time takes on the characteristics of a streaked fibrous connective tissue, 

 and becomes compacted. The degeneration of the entoderm is apparent 

 not only in the inner layer which lines the cavity of the vesicle, but is 

 seen especially well in the groups of entodermal cells which lie scattered 

 throughout the mesoderm. In many cases the entodermal cells are rep- 

 resented by granular detritns without any remnants of nuclei, while in 



3 — mesoderm 

 ^bloodvessel 



Fig. 5. Umbilical vesicle from an embryo 7 mm. long (No. 18). X 25. 



other cases the cell outlines are faintly seen, and the nuclei are well pre- 

 served. Large amounts of cellular detritus can be found in the cavity 

 of such a vesicle, and it does not seem unlikely that the cell remnants 

 found among the calcareous contents of vesicles taken from placentae at 

 birth have this origin. This cellular detritus is especially well seen in 

 Nos. 187 and 176, the cavities of which vesicles are almost completely 

 filled with granular debris containing many large cells having the charac- 

 teristics of entodermal cells. In older vesicles, those from embryos of 

 sixty and one hundred millimeters, for example, we find, on the con- 

 trary, a condition almost identical with that found in full-term vesicles, 

 except that the walls of the latter are more compacted and look still more 

 as though composed of mature fibrous connective tissue. 



The signs of degeneration in these older vesicles are not limited to 

 the entoderm, however, for many of the blood vessels show marked degen- 

 eration of their walls and of the nucleated red blood cells contained 

 within. The vessels are often pigmented and without a proper lining. 

 The pigment reminds one strongly of blood pigment and looks very 

 much indeed like hsematoidin. The entire absence of vessels in the old 



