6 CHARLES R. STOCKARD 



cells really differing in nature according to whether they will 

 give rise to one or the other of the three cell types. Yet they 

 may not differ from one another in any way by which we can 

 at present distinguish them. If this proposition be true, or even 

 if the weight of evidence lean in this direction, it is scarcely 

 more justifiable to derive these completely different cells from 

 a common mother cell than it would be to derive connective 

 tissue and blood cells from a common mother cell. 



Of course, we are only considering the mesenchymal cell just 

 before its differentiation is to begin. Carried back further, no 

 doubt, all the cells become more and more alike and possess 

 more and more complex potentiahties as is so thoroughly dem- 

 onstrated by the numerous studies of cell lineage. In the be- 

 ginning, of course, all cells arose from one single egg cell capable 

 of giving rise to every tissue of the body, but after tendencies 

 in differentiation have proceeded sufficiently far in the various 

 cells some then form real mesenchymal cells. Later individual 

 mesenchymal cells incline in certain directions and finally be- 

 come incapable of giving rise to any other than the definite type 

 of tissue or cells towards which their particular tendencies have 

 directed just as certain endodermal cells become specialized to 

 form the liver while others near by and at first indistinguishable 

 from these give rise to the ducts and acini of the pancreas. 



All of the vertebrate classes present these many questions 

 of blood origin, etc., but the forms upon which this investiga- 

 tion has been conducted, the Teleosts, possess in addition 

 many extremely interesting special problems. In all other 

 meroblastic embryos the majority of the earliest blood cells arise 

 in yolk-sac blood islands. Yet in many of the Teleosts there are 

 apparently no early blood islands on the yolk, but all of the 

 blood forming cells are contained within the embiyonic body. 



This intra-embryonal blood anlage has been frequently de- 

 scribed by many authors as the "intermediate cell mass." The 

 intermediate cell mass as has been suggested by Marcus ('05), 

 Mollier ('06), and others, is really the homologue of the blood 

 forming yolk-sac mesoderm in the other meroblastic types. 



