10 HENRY MCELDERRY KNOWER 



amount of material for comparative study of different em- 

 bryos in amphibia and fish; and 2) even more important, to 

 push the injections back into stages exhibiting the very begin- 

 nings of the lymphatic system. 



In order to accumulate plenty of the desired material, I 

 have made hundreds of injections (single and double) of both 

 blood vessels and lymphatics in young larvae, with the aid of 

 the special micro-injector of 1908, using the following forms : 

 five species of frog, E. sylvatica, R. catesbiana, E. clamitans, 

 R. pipiens and E. palustris ; five urodeles, Amblystoma punc- 

 tatum, A. tigrinum, Euricea, Necturus and a pigment-free 

 axolotl ; also two species of fish, toadfish and trout : all forms 

 offering comparative data for understanding the conditions 

 in the frog. 



THE FIRST LYMPHATIC STAGE 



When lymphatics first appear in injections of frog embryos, 

 a complete blood circulatory system is already established in 

 simple form, distributed to the primary organs, as in the cor- 

 responding fundamental stage of vascular development in 

 other vertebrate embryos. These conditions are illustrated 

 in the beautiful pictures and full accounts of Evans ( '09), for 

 birds and mammals; Sabin ('17), for the same forms; Heuser 

 ('23), for mammals; Grodzinski ('28), for lower forms. 



A review of the characters of this fundamental stage in the 

 frog will be introduced, not only to facilitate the study of 

 differentiations of blood vessels in the plexuses, but also to 

 furnish definite pictures not found available in a form to 

 permit comparisons with other vertebrate embryos. Such 

 pictures are also lacking for the recognition of lymphatics 

 which invade, in the early period, the various vascular and 

 non-vascular areas mapped out by the injection. 



If the descriptions of the vascular system presented for 

 injections of the first stages appear somewhat lengthy, this 

 is because the veins are already connected up definitely in 

 different regional systems which require full details for satis- 

 factory orientation of the related lymphatics still incompletely 

 distributed. 



