they are identical in appearance v/ith the amoeboid cells 

 which are scattered about abundantly in the body-cavities of 

 tne buds and the sinuses of the stolon. As I have emphasis- 

 ed repeatedly in describing the formation of tne various or- 

 gans in question, there is not the slightest discoverable ev- 

 idence that cells are given off from the wall of the inner 

 vesicle at the points where such organs arise, or at any oth- 

 er place, for that matter, and this fact, together with the 

 similarity betv/een the free mesodermal cells and the cells 

 which go to make up these rudiments, is strong proof that the 

 latter are genuine mesodermal cells. 



The origin of these cells is a question which is 



practically undeterminable, and direct proof that they are 



of tne mesoderm 

 all descendants of the larva, is of course impossible, al- 



though the assumption that they are, seems to me to be a jus- 

 tifiable one. The statement that they do not arise from 

 cells which wander out from the endoderm or ectoderm, is ba- 

 sed, it is true, on negative evidence, since after the most 

 diligent search it cannot be discovered that any are derived 

 in this manner. 



It is, perhaps, going a little far to absolutely 

 deny such an origin, for it would be impossible from the na- 

 ture of the case to say that at no time does the endoderm or 

 ectoderm give off cells to the blood, even although it cannot 

 be shown that such is the fact. 



The positive evidence furnished by their mesodermal 

 appearance, however, together with the negative evider. ce de- 



