MYELOID METAPLASIA OF THE EMBRYONIC MESENCHYME. 9 



poiesis has so far been described in the normal allantois, but the changes displayed 

 under experimental conditions by this membrane, with its special vascularization, 

 can not serve as a basis for deductions applying to other parts of the host's mesen- 

 chyme. One can not therefore be fully convinced, on the ground of the results 

 already described, that all of the loose mesenchyme found in the embryo at about 

 the seventh to the eighth day of incubation is polyvalent as well as equipotential, 

 at least in regard to blood-forming power. A manifestation of various hemopoietic 

 activity by all the mesenchyme of the whole embryonic body can be advanced as 

 only valid proof of the equivalency of the mesenchyme in general, of the existence 

 of latent potencies in the mesenchyme and of its polyvalency. 



At the meeting of the Anatomical Society in New Haven in 1915 (19166) I 

 had an opportunity of demonstrating granulopoietic activity in the mesenchyme 

 between the muscles and groups of muscular fibers in different regions of the 

 embryo after grafts of adult splenic tissue on its allantois. In a later paper (1916c) 

 I stated that "no place exists in the embryo body where (under experimental con- 

 ditions) the mesenchyme does not show this process. Thus, the embryonic mesen- 

 chyme appears to be a diffuse anlage for both lymphopoiesis and granuloleukopoi- 

 esis." Since then additional data relating to the sex-glands, the kidneys, and other 

 organs have accumulated, and finally it became obvious indeed that there re- 

 mained no place in the body of a chick embryo of 7 days' incubation in which 

 the loose mesenchyme could not be incited to granuloblastic differentiation. These 

 additional data, together with those occasionally mentioned or demonstrated, 

 will now be presented under separate paragraphs. 



MESENCHYME OF THE MUSCLES. 



The scanty mesenchyme within a muscle under normal conditions is invariably 

 transformed into connective-tissue cells only. It could not be otherwise. The 

 development of fibroblasts is here the result of interaction of the definite physico- 

 chemical constitution of the mesenchymal cell plus definite normal environmental 

 conditions. However, the mere observation that the mesenchymal cells in the 

 muscles under the same normal conditions invariably develop into fibroblasts does 

 not allow us as yet to conclude that these cellular elements are specific or univalent ; 

 i. e., that they would, under any non-injurious conditions, develop into fibroblasts 

 only. It is true that investigations on muscle regeneration did not reveal new poten- 

 cies in the mesenchymal constituents of the muscles, but a negative result obtained 

 under definite experimental conditions can not be immediately interpreted as proof 

 of an absolute lack of a potency in these cells or in their ancestors. The study of 

 the mesenchyme of developing muscles after grafts of adult splenic tissue on the 

 allantois of the embryo brings a new evidence for the existence of latent blood- 

 forming potencies within a part of the embryonic mesenchyme previously unsus- 

 pected of possessing such a potency. The loose mesenchyme between groups of 

 muscle fibers and the mesenchyme around the different muscles in the neck region, 

 in the wings and legs, as well as the connective-tissue cells around the tendons, show 

 after this intervention an extensive granulopoietic transformation. 



