132 CHARLES R. STOCKARD 



It would seem that these cells must have some potential 

 differences at the time they come to lie in the yolk-sac, since 

 from that tune on they all appear to be in an identical environ- 

 ment. Two cells lying side by side in the yolk-sac above the 

 periblast and beneath the ectoderm would be expected to de- 

 velop and grow in similar fashion unless there were some internal 

 difference between them. I have thus concluded that the mesen- 

 chymal cells which wander in the yolk-sac of the Fundulus embryo 

 must be potentially of four different classes when they first wan- 

 der out, although all have the ordinary appearance of embryonic 

 mesenchymal cells. Otherwise, it is difficult to conceive why 

 they should develop into four distinct types of cells while all 

 are surrounded by an identical environment so far as is possible 

 to discover. Differentiation in various directions must be due 

 either in the first place to similar cells developing in different 

 chemical or physical surroundings, or in the second place it may 

 result from potentially different cells developing under identical 

 conditions. 



The four types of cells are all derived from mesenchyme, just 

 as the thyroid follicles and pulmonary epithelium are derived 

 from endoderm but from different endodermal anlagen, and 

 further than this there is no relationship. Pigment cells and 

 blood corpuscles are perfectly separate and distinct types derived 

 from different mesenchymal analgen and are not in any way 

 transmutable. 



2. History of the endothelial cells 



The endothelial cells on the living yolk-sac of Fundulus em- 

 bryos are readily recognized. Their entire behavior in the for- 

 mation of the earliest yolk vessels may be traced in a manner to 

 fully repay the patient observations necessary in order to follow 

 through the processes. 



Among the early wandering cells migrating away from the 

 lateral borders and caudal end of the embryo it is noted that 

 certain ones assume a delicate spindle shape with filamentous 

 processes extending from their ends and occasionally projecting 



