234 BULLETIN OF THE 



smaller than in the cells of the tubules. Very similar elements abound 

 in the fundaments of blood-vessels at this stage, and it is evident that 

 the cells are embryonic blood corpuscles. The spaces in which they 

 occur constitute a complicated system of communicating blood sinuses, 

 and are continuous with the lumens of the vessels entering and leaving 

 the pronephros. 



The other class of cells to which I have referred are mesenchymatic. 

 I have carefully studied these cells in the endeavor to ascertain their 

 precise origin. A mode of reasoning similar to that employed in dis- 

 cussing the probable origin of the inner cells of the glomus leads to 

 the conclusion that the mesenchymatic cells of the pronephros cannot 

 have been given off from the walls of the tubules. As I have stated, 

 the cells in these walls are very thick, and their nuclei lie close to the 

 lumen of the tube. Under these circumstances, it is difficult to under- 

 stand how any cells of the tubule should divide so as to give off from 

 their basal surfaces cells as small as those in question. The usual pro- 

 cess of cell division, if it took place parallel to the surface of the layer, 

 would result in the production of a small cell on the side toward the 

 lumen and a large outer segment. Such a large cell might, it is true, 

 by repeated divisions, break up into numerous small cells, but for several 

 reasons I do not believe this to have been the case. If such a delamina- 

 tion and subsequent cell division took place, it would naturally be a 

 conspicuous process ; but I have never observed any evidences of it. 

 This method of origin would involve a considerable thinning of the tubes, 

 which does not take place. 



There remain two other possible sources for the mesenchymatic cells 

 of the pronephros. They may have arisen from the tissue bounding 

 the pronephros, viz. the capsular membrane and the adjacent somato- 

 pleure, or they may have come from remote regions. In judging 

 between these possibilities, it is important to consider the sudden 

 appearance of the cells and the small amount of differentiation they 

 have undergone. It seems to me highly improbable that they should 

 have already accomplished any extensive migrations. Under these cir- 

 cumstances, such positive evidence as I am able to adduce is the more 

 convincing. In studying the youngest stages in which mesenchyme 

 was preseut in the pronephros, this tissue was usually found near the 

 somatopleure or the capsule, and frequently consisted of a row of cells 

 closely applied to one of these layers. Occasionally I have seen a layer 

 of mesenchymatic cells arranged along the somatopleure in a very definite 

 manner, so that the nuclei of the two layers lay directly opposite each 



