162 The Structure of Human Umbilical Vesicle 



long diameter of the vesicle, for only occasionally was a tubule cut at 

 other than a slight angle to its long diameter. Even when such was the 

 case it could generally be accounted for by the fact that the plane of the 

 microtome knife was not at right angles to the long diameter of the 

 vesicle. 



In spite of the large amount of material at my disposal, I am unable 

 to reach any satisfactory conclusion as to the meaning of these tubules. 

 Their presence is not at all a constant one. Vesicles of the same age 

 and size often present wide divergencies of structure which are hard to 

 reconcile. I feel justified, however, in suggesting an explanation of the 

 manner of formation, which an examination of the material at my dis- 

 posal will, I think, corroborate. Two methods of formation can be 

 distinguished: (1) evagination of the entoderm and (2) development 

 from irregular extensions of entoderm into the mesoderm. That the 

 first step in the formations of many tubules is a slight evagination of 

 the entoderm, as Graf Spee has stated, is very evident. I have found 

 all transitions between such a stage and perfect tubules lying isolated in 

 the mesoderm. This isolation can be readily brought about by a gradual 

 deepening of the original evaginations accompanied by constriction and 

 consequent fusion. This process seems to be further indicated by the 

 occurrence of tubules which communicate with the cavity of the vesicle 

 by their ends only, while others are closed at both ends and lie isolated 

 in the mesoderm close to the entoderm. It seems highly probable to me 

 that an active proliferation of the mesoderm might play a part in this 

 separation of the tubules and their further removal to the periphery of 

 the mesoderm. 



Even if correct, however, this explanation cannot account for those 

 tubules in whose lumina masses of unmistakable mesoderm are found. 

 This is the case in No. 33, an embryo twenty millimeters long. In this 

 specimen there are striking evidences of the formation of tubules by 

 proliferation from irregular extensions of entodermal cells. Such in- 

 clusions of mesoderm might evidently result from tubule formation by 

 invagination of the entoderm, but it is hard to find any satisfactory 

 evidences of such a process of inclusion. That another method than 

 that of evagination of the entoderm must have been followed, however, 

 in the case of No. 32, is clearly indicated not only by the masses of 

 mesoderm contained in the tubules, but especially by the fact that 

 strands of mesoderm are found in various stages of inclusion by the 

 entoderm. That an active proliferation of the entoderm into the meso- 

 derm does occur, is further indicated by those specimens in which almost 



