160 



The Structure of Human Umbilical Vesicle 



Graf Spee is well seen in a vesicle taken from an embryo thirteen milli- 

 meters long. In such a vesicle (Fig. 4) we find an almost complete canal- 

 ization of the mesoderm while the entoderm is but little changed. The 



;■ 7— mesoderm 



-?^^^lubule 



Fig. 2. Umbilical vesicle of an embryo 7 mm. long (No. 2). X 35. 



tubules are much larger and longer and are formed by a layer of flat cells 

 which often approach the cubical type. Contact of tubules is common but 

 definite branching is infrequent. The lumina are wide and contain 



Fig. 3. Tubules from the vesicles of an embryo 7 mm. long (No. 2). X 35 ; (a) 

 Simple evagination of entoderm — first stage; (5) Same, second stage; (c) Isolated 

 tubule. 



confused masses of amorphous material similar to that found in the 

 cavity of many of the younger vesicles. They never seem to open directly 

 into the cavity of the vesicle, although often the entoderm only separates 

 their lumina from it. They are of many sizes, shapes and lengths, and 

 lie irregularly distributed in the mesoderm. When not in contact they 



