Arthur W. Meyer 



161 



often have irregiihir masses of entoderm between them or are separated 

 by mesoderm. Their abundance gives a striking appearance to sections 

 of the vesicle which is well expressed by Graf Spee as " ausserst liick- 

 enreich." It is worthy of note that the lumina of the tubules have 

 greatly increased in diameter while the thickness of the bounding endo- 

 theliimi has, absolutely as well as relatively, decreased. In many cases 

 the shape of the individual cells also has changed from cubical or pyram- 

 idal to a membranous-like layer of greatly flattened cells. 



In older vesicles these tubules occur but rarely. This is usually the 

 case in vesicles of the ninth and tenth weeks, although one vesicle taken 



Fig. 4. Umbilical vesicle from an embryo 13 mm. long (No. 175). X 2.5. 



from a normal embryo of the fifth week has already reached the stage 

 of those three or four weeks older. Generally these older vesicles have 

 a very different structure than those of four or five weeks and contain 

 masses of calcareous matter. 



It seems then that these tubules make their appearance during the 

 second week, reach their greatest development by the fourth or fifth week 

 and then gradually disappear by the eighth or ninth week. These 

 stages are well represented in embryos Nos. 11 and 12; 113 and 175; and 

 145, 176 and 184 respectively. This conclusion is at variance with the 

 observation of Graf Spee on embryo Gle, but as the widest variations as 

 to the presence, structure and size of these tubules exist the contradiction 

 does not seem surprising. As a rule the only constant characteristic 

 was their direction. This was almost invariably in the direction of the 



