164 The Structure of Human Umbilical Vesicle 



vesicles and their extreme vascularity in the early stages alone seem 

 sufficient to indicate a gradual degeneration. 



The walls of these vesicles, as already stated, vary greatly in thickness 

 and in the character of the cells composing them (Figs. 2, -i, 5). Usu- 

 ally the greatest thickness is found at the distal end. Both entoderm 

 and mesoderm are present in all vesicles except that of No. 187, below 

 eight weeks of age. In these specimens the ccelomic epithelium in addi- 

 tion extends over the entire surface of the vesicle. This envelope is 

 invariably composed of a single layer of very much flattened cells with 

 elongated nuclei. 



The mesoderm also presents great variations in thickness, though not 

 in the character of its cells. These cells, though cuboidal or cylindrical 

 in a few instances, not infrequently look like embryonic connective-tissue 

 cells in the young vesicles, while in those of ten weeks and older it has 

 the characteristics of fibrous connective tissue, as already noted. In 

 these specimens it is denser, and stained more deeply near the cavity of 

 the vesicle. The tubules and blood vessels invariably lie in the meso- 

 derm, but are frequently surrounded by extensions or by groups of ento- 

 dermal cells. In younger vesicles the blood vessels and blood islands 

 usually cause an elevation of the mesoderm above the points where they 

 lie. 



The entoderm is composed of a single layer of cuboidal, pyramidal, 

 and exceptionally in a small area, of cylindrical cells in vesicles of two 

 to four weeks, but is absent in those over seven weeks of age. In a few 

 specimens no distinct demarcation between entoderm and mesoderm can 

 be found, though usually they are clearly defined in all the younger 

 vesicles (Fig. 1). 



A series of six imibilical vesicles taken from placentae at birth were 

 found almost identical in structure with tlxe vesicles of JSTos. 184, 171 and 

 X. The walls of these vesicles are composed of a dense, wavy layer of 

 fibrous connective tissue of varying thickness, which blends more or less 

 with amnion and chorion. The cavity contains an irregular mass of 

 calcareous matter among which cell remnants are plainly visible. Even 

 those vesicles which are inflated sacs contain a small amount of calcareous 

 matter, while those which are compressed and irregularly folded contain 

 a firm mass of calcareous substance, which completely fills the cavity of 

 the vesicle. Eemnants of the early blood vessels or of tubules are never 

 found nor can any recognizable remnants of the entoderm be detected. 

 Unless, as previously suggested, the cells lying among the calcareous 

 matter have this origin. The striking similarity between the structure 



