GROWTH OF BLOOD-VKSSKLS IN FROG LARVAE 81 



servation made by Knower, and confirmed by myself, that there 

 is movement of lymph, as shown by the passage through the 

 l>'ni])h h(>art of an occasional blood-cell, for it shows that passage 

 of lymph into lymphatics is not dependent upon the main- 

 tenance of a certain amount of l^lood pressure. 



The studies on embryos without circulation show that without 

 the action of the mechanical factors concerned with circulation 

 an extensive development of blood-vessels takes place ; that some 

 vessels — at least the aorta in chick and fundulus embryos — dif- 

 ferentiate bej^ond the capillary stage. My finding that, in such 

 embryos, growth may occur by the usual process of sprouting, 

 indicates that this property of endothelium is not dependent for 

 its earh^ manifestation upon the action of the specific mechanical 

 or chemical factors which seem to regulate it in later stages. 80 

 far as they bear on the main problem of this investigation, these 

 results are important as indicating the extent and character of 

 development of the vascular system which may take place with- 

 out the mechanical factors concerned with the circulation of 

 blood. It has been clearly brought out, especially by Roux, 

 that there are two chief stages in the development of each organ 

 or tissue. The first stage is the stage of 'auto-differentiation,' in 

 which, by virtue of what our ignorance forces us to call heredity, 

 or as Noel Paton expresses it, 'hereditary inertia,' each tissue dif- 

 ferentiates and develops to a certain point. The second stage 

 is the stage of 'functionelle anpassung,' functional adaptation, in 

 which the further grow^th is regulated mainly by factors con- 

 cerned in a quantitative way with the especial function of the 

 organ or tissue. It is, therefore, consonant with our knowl- 

 edge of many other organs and tissues to find that blood- 

 vessel endothelium differentiates and grows for a certain period, 

 and even that a vessel such as the aorta develops, as the results 

 of 'heredity,' and without the action of mechanical forces. Such 

 a finding affords no objection to the thesis that, in their later 

 growth, blood-vessels are subject to the regulative action of the 

 moving blood stream, the blood-pressure, the mechanical ten- 

 sion exerted on the wall by outside tissues, and the amount of 

 passage of substances through the vessel wall. In fact, were it 



