DEVELOPMENT OF WANDERING MESENCHYMAL CELLS 167 



On the other hand, when Thoma has a straight normal case, 

 the lumen may readily be seen to increase in size with the rate 

 of flow. Yet in the entire absence of this action the vessel 

 is still capable of increasing in size and it becomes question- 

 able whether the rate of flow is ever an actual cause of size 

 increase beyond mechanical stretching. 



These facts are most significant in a consideration of the 

 influence of function on growth and development, auto-differ- 

 entiation. Here it is seen that the structure both grows and 

 develops in entire absence of its function. In normal cases the 

 function of the vessel as a blood conductor exerts more likely a 

 physical rather than a biological effect on development. 



Thus the development of blood vessels on the yolk-sac of the living 

 Fundulus embryo proves that the capillaries are not universally 

 the anlage of the arteries and veins, but that these larger vessels may 

 arise directly from wandering mesenchyme cells. Such arteries 

 and veins may grow and persist without a circulation of the blood 

 through their lumen and even though stagnant masses of corpuscles 

 may crowd the vessel cavity. 



The development of vessels in Fundulus also directly disproves 

 the claims made by Sobotta ('02) that the vessels in the teleost 

 grow over the yolk entirely as branches from those near the 

 embryo and without the wandering cells taking part. This 

 assumption is probably due to the diflticulty of estimating the 

 part played by wandering cells from a study of serial sections. 

 From a study of the living yolk-sac there is no question of the 

 major part played by the wandering cells* in the origin and 

 formation of vessels on the yolk. 



Sobotta also advances the opinion that the entire yolk vessels 

 may sprout from the heart. This is much of the same nature 

 as the ingrowth or parablast theory of His (75), and is obviously 

 defeated by the same array of facts which long ago relegated the 

 parablast theory to a place of mere historic interest, in spite of 

 the fact that it is so often revived for literary reasons. 



Finally, we may consider the study of the developmental 

 products of the early wandering mesenchymal cells on the yolk- 

 sac of the Fundulus embryo as a problem of cell lineage carried 



