54 CHARLES F. W. McCLURE 



entiation from mesenchyme is completed, forms no exception to 

 other differentiated tissues in its general ability to grow. As 

 endothelium naturally cannot grow, as such, until it has been 

 formed, its local genesis from mesenchyme must be regarded as a 

 process distinct from that of its subsequent growth. Both proc- 

 esses combined, however, constitute its development. 



The theory of the local origin of endothelium from mesenchyme 

 was originally based on purely morphological observations and 

 not on experiment. Now that experiment has confirmed this view 

 we are doubly assured of the accuracy of our morphological 

 observations, as well as of their interpretations. The identical 

 morphological observations so uniformly made for the local 

 origin from mesenchyme of lymphatic and blood-vascular chan- 

 nels, can therefore leave little room for doubt, as regards a cor- 

 responding identity in their mode of development. 

 As previously stated by the writer ('15, J 15b): 

 The development of the general vascular system, both haemal 

 and lymphatic, is a uniform process which consists in a local 

 origin (genesis) of endothelium from mesenchyme, and a growth 

 of endothelium after it has once been formed. 



13. THE PROGRESSIVE REACTIONS OF THE TISSUES TOWARD COL- 

 LOIDAL ACID DYES AS OBSERVED IN THE COURSE OF 

 LARVAL DEVELOPMENT 



The progressive reactions of the tissues toward colloidal acid 

 dyes, as observed in the course of larval development, is a ques- 

 tion of interest and importance. If these reactions are indicative 

 of the functional activity of the tissues, one should be able to 

 determine approximately when certain organs first begin to 

 function in the embryo. In addition to the pronephros, as ob- 

 served at the critical ontogenetic stage (p. 13), the organs which 

 lend themselves especially to such an investigation are the 

 mesonephroi. The spleen, thymus gland, bone-marrow and the 

 different types of white blood-corpulses might also possibly be 

 studied in this manner. 



In studying the reactions of tissues toward acid dyes, as ob- 

 served at any particular stage of development, it should be borne 



