Hemopoiesis in the Mongoose Embryo. 305 



processes are still largely in theory. The bulk of the best evidence, 

 however, seems to be in favor of a monophyletic genetic method. The 

 above observations are in accord with, and in a degree a support to, 

 the monophyletic theory. The evidence shows that young endothe- 

 lium and mesothelium are only slightly altered (mechanically) mesen- 

 chyma, and that the two tissues may in early stages give rise to 

 hemoblasts. In the young embryo these hemoblasts differentiate into 

 erythrocytes. The primitive leucocyte ("lymphocyte") is represented 

 by this hemoblast; in later stages this hemoblast (leucoblast) may 

 differentiate into the various types of granulocytes. 



The monophyletic theory rests, then, essentially upon these three 

 facts: (1) the origin of the early hemoblast from mesenchyme, now 

 no longer disputed; (2) the differentiation of these hemoblasts into 

 erythrocytes, also no longer disputed; (3) the identity, inferred from 

 a very close structural similarity, of hemoblast and lymphocyte, which 

 latter is regarded as the progenitor of the granulocytes. The third 

 point is the one on which the discussion and dissension center. Grant- 

 ing the verity of the third point, it follows that the primitive leucocytes 

 appear before the erythrocytes. Such an ontogenetic sequence is in 

 accord with the principle of progressive differentiation and with the 

 phylogenetic history a very significant fact. 



The sharpest attack upon the monophyletic theory of recent years 

 has been made by Stockard (21), on the basis of his observations on 

 Fundulus embryos; but his more definite and positive facts, namely, 

 the origin of the erythroblasts, "leucoblasts," and endothelium from 

 mesenchyma as seen in the living embryo are actually in accord with 

 the monophyletic theory and furnish strong additional support. The 

 observation upon which he takes foothold to launch his attack, namely, 

 the segregation of erythropoietic and leucopoietic foci, is open to the 

 criticism of misinterpretation with respect to the latter. At any rate, 

 his illustrations of primitive "leucocytes" do not inspire confidence; 

 the karyorrhexis shown would alone make one suspicious of degenera- 

 tive phenomena, and their general appearance raises the question 

 whether they are not actually (as also Reagan (ie) suggests) degenera- 

 ting erythroblasts suffering nuclear changes and hemolysis. 



ADDENDUM. 



The above was ready for publication in essentially the form here 

 given early in March 1916. Meanwhile Emmel published a detailed 

 description of the aortic cell-clusters in pig embryos (Amer. Jour. 

 Anat., vol. 19, 3, 1916) and announced the publication of a paper 

 "Concerning certain cellular elements in the coelomic cavities and 

 mesenchyma of mammalian embryos" (Amer. Jour. Anat., in press 

 for vol. 19). After reading the first of these papers it seemed to me 

 better to note and discuss them in an addendum than to rewrite 



