MYELOID METAPLASIA OF THE EMBRYONIC MESENCHYME. 5 



of similar experiments, came to the conclusion "that the essential process of differ- 

 entiation, whereby the potency to form a forelimb becomes localized in certain 

 cells of the body wall, must be relegated to very early embryonic life." 



But there are a number of experiments the results of which indicate that other 

 primordia in other animals may possess a polyvalency during the embryonic 

 stages and even throughout life. Numerous cases in the vast domain of regenera- 

 tion can be explained only on the basis of a polyvalency retained by certain struc- 

 tures. Various cases of heteromorphosis (including the transformation of the 

 epithelium of the surface body into a lens) suggest strongly the existence of poly- 

 valent structures. The regeneration in crustaceans of an eye or an antenna in the 

 place of an extirpated eye (according to whether the optic ganglion was removed 

 or left in place) is one example among many that could be cited (Herbst, 1895) . Of 

 course, in the cases of regeneration of various complex organs the advocates of 

 tissue specificity, if microscopical study of the experimental material is omitted, can 

 always advance the suggestion that there exists a specific primordium for every 

 organ that might arise from a definite part of the organism. 



My personal experience in the study of the primordia of hemopoietic organs, 

 and of their activity in a normal adult organism as well as under pathological con- 

 ditions, does not allow me to extend the conception of specificity or univalency to 

 all the primordia or even to the tissues of the developing embryo. No doubt the 

 different organs, under normal conditions, take a predetermined line of develop- 

 ment. This is a definite and invariable result formulated by descriptive embry- 

 ology. However, their actual development is the result of complex factors, which 

 in part are represented by the constitution of the primordium itself, either poly- 

 valent or univalent, and in part by factors lying outside of it. The mere obser- 

 vation that the primordia of the embryo under the same normal conditions develop 

 into the same organs does not justify us in concluding that they are univalent and 

 self-determining. Freed from a possibly inhibitory influence of their regular en- 

 vironment and submitted to other conditions, they might have manifested other 

 potencies. If so, their normal development would have been determined, or at 

 least limited, not by intrinsic factors consisting of a specific constitution of the 

 primordia, but by factors external to them; the latter, therefore, would have deter- 

 mined which of their potencies were to be realized, and to what extent. 



Most of the primordia of the organs in a developing embryo are localized and 

 enter into definite correlations with other organs. Though limited to the mesen- 

 chyme, the hemopoietic differentiation is not confined to definite topographic 

 boundaries. It is true that at an early stage of embryonic development blood 

 primordia appear in the form of blood-islands in the area opaca and develop there 

 from the mesodermal mesenchyme, but this is not the only place where hemopoietic 

 tissue develops. Mesenchyme differentiates into blood-cells in the area opaca, 

 between the growing buds of the liver-cells, in the spleen, in the bone-marrow, in 

 many places along the intestinal tube, and (in a more diffuse manner) in the loose 

 tissue between different organs. At a certain stage of embryonic development the 

 diffuse hemopoiesis ceases and in a normal adult organism it is well localized. 



