ORIGIN OF BLOOD AND ENDOTHELIUM 305 



somites and contains both the somatic and splanchnic layers 

 of mesoderm. The origin of the aorta from the 'mesenchymaor- 

 tenstrang' is from the same cell material as the mesodermal 

 layers. The cavity of the myotom is secondary body cavity, 

 coelom, and so also is the aortic cavity. Neither is in any way 

 primary body cavity. The formation of the aortic cavity is 

 a similar process to the canalization of the stammvene. Felix 

 in this way arrives at a conclusion diametrically opposed to 

 Ziegler. 



These conclusions he recognizes are not facts but are based 

 on facts obtained from a study of Teleosts which are a side 

 branch of the vertebrate stem, but from which one may still 

 generalize to some extent. Felix calls attention to the fact that 

 in the selachians Zeigler ('92), and in the reptiles Strahl ('83, 

 '85), and in the birds Kolhker ('84) and Ziegler ('92), and in the 

 mammals Kolliker ('84), all claim that the first vessel anlagen 

 are found in the mesoderm and not between the mesoderm and 

 endoderm. Only in the mesoderm the secondary body cavity 

 arises by splitting, and since the solid vascular anlagen are 

 formed within the mesoderm their cavities should not be considered 

 primary body cavity. The writer is entirely unable to agree 

 with such an analysis of the origin of vessels, particularly yolk 

 vessels, as well as of the primary and secondary body cavities 

 for reasons given below. 



Felix ('97) now goes further and assumes that the lymph 

 vessels arise in mesenchyme and their cavity is primary body 

 cavity and their wall cells are modified connective tissue cells. 

 This position is difficult to appreciate since it must be admitted 

 that mesenchyme is a direct product of the mesoderm, and, 

 according to Felix, any definitely formed cavity arising between 

 such cells would seem to be coelom. I question, however, 

 whether any other morphologist would put the same interpreta- 

 tion on all the spaces cited by Felix as being in the coelomic 

 category. Felix states, for example, that the aorta arises from 

 a mesenchymaortenstrang derived from the sclerotom. The 

 sclerotom is more or less mesenchymal in nature and certainly 

 contains many cells which will later give rise to types of con- 



