ORIGIN OF LYMPHATICS IN BUFO 163 



tissues. Of course this does not necessarily imply that certain 

 tissues are more active than others and use up the stored nutri- 

 tive supply more rapidly. On the contrary, some may be more 

 richly supplied at the outset. Thus organs derived directly 

 from entoderm retain traces of yolk much longer than others. 

 These considerations suddenly led to the inspiration that the 

 presence of yolk might after all prove to be an advantage since 

 it might point out the relationship of lymphatic endothelium. 

 Such a natural and simple factor, if found and capable of being 

 utilized as an instrument of proof, would carry far more weight 

 than any possible evidence obtained by the use of special stain- 

 ing processes, injections, or other artificial procedure. A critical 

 study of the behavior of the several tissues in this respect has 

 convinced the writer of the soundness of this principle. Refer- 

 ring especially to the head region, which in vertebrate embryos is 

 ever in advance of other parts of the organism in degree of devel- 

 opment, it was observed that the mesenchymal cells dispose of 

 their yolk content much earlier than do blood corpuscles and 

 vascular endothelia, both haemal and lymphatic. To be more 

 specific, in 6 and 7 mm. embryos of the European common toad 

 the cephalic mesenchyme is virtually destitute of yolk, while 

 the lining of the lymphatic anlagen contains large globules even 

 in specimens measuring 9 mm. in length. To the author's 

 mind these facts indicate fundamental differences between the 

 two tissues and show that in Amphibia, at least, the lym- 

 phatic intima arises not by direct differentiation of mesenchymal 

 elements. The idea of similarity of origin but diversity due 

 to function cannot be urged against this argument, for at the 

 time when the yolk-filled rudiments of the large cranial lymph 

 channels appear the embryonic connective tissue of the same 

 region has lost most of its yolk granules. 



Before entering into an interpretation of the appended figures 

 illustrating the source of certain lymph vessels, a brief descrip- 

 tion of a later embryonic phase, when an effective system of lym- 

 phatics is already in existence, is essential to the proper under- 

 standing of the narrative. This can best be done by referring 

 the reader to Hoyer's figure 417 in the seventh edition of Wieder- 



