THE CELL LAYERS 311 



that which is of real importance in His's observations, namely, 

 the recognition of the ultimate evolution of the primitive meso- 

 blast into two distinct series of cellular constituents. 



For our purpose it is unnecessary to choose definitely between 

 the two main contending views as to the origin of mesenchyme. 

 Whether all the primitive mesoblast first passes into a mesothelial 

 stage, from which by further proliferation the mesenchyme is 

 derived ; or whether, on the other hand, a portion of the meso- 

 blast does not undergo conversion into mesothelium, but con- 

 tinues directly to develop into mesenchyme, is for us a relatively 

 secondary matter. The important point is that we have to 

 recognize that the primitive mesoblast is eventually separated 

 into these two sets of cells, and of these the mesothelium is 

 differentiated into a layer of the lining membrane type. 



At a still later date masses of mesothelial cells again accumu- 

 late, and, as was the case with the epiblast and the hypoblast 

 in the earlier stage, they give off on either side a mass of more 

 undifferentiated cells, and these masses form the mother tissue 

 or Anlage of the eventual striated muscle. Later, though still 

 in this embryonic period, with the development of the first 

 vessels, the mesenchyme gives off a series of cells of the lining 

 membrane type, which form the eventual lining cells, or endo- 

 thelium, of the vascular and lymphatic systems. 



There is still some little uncertainty as to the exact relation- 

 ship of the vascular endothelium, whether it be directly derived 

 from mesothelium or from the mesenchyme. As Professor 

 Minot has pointed out to me, His has of late indicated that it 

 is of relatively very early development in certain forms. On 

 the other hand, I learn from Professor MacBride that relatively 

 high up among the forms of animal life it may be wanting, as, 

 again, it may only show itself at a period definitely later than 

 the development of the vascular channels. What I have stated 

 above, thus, may be taken as representing, as accurately as is 

 possible at the present time, the generally accepted relationships 

 in time of endothelial to other embryonic developments. 



Thus during early embryonic life we obtain a series of differ- 

 entiations of the primitive cell layers leading to the production 

 of two sets of tissues ; one which we may term the lining mem- 

 brane tissues, the other the pulp tissues. I do not wholly like 

 the latter expression, but can think of none other which more 



