6 MYELOID METAPLASIA OF THE EMBRYONIC MESENCHYME. 



While all the mesenchyme found between different organs of a normal embryo 

 does not by any means differentiate into blood-cells, the mere fact that invariably, 

 under normal conditions, only a part of it develops into blood-cells is certainly not 

 a sufficient reason for assigning to that part certain specific properties not present in 

 the mesenchyme of other regions. Other mesenchymal cells, except those which 

 actually develop into blood-cells, may possess analogous potencies, but by special 

 conditions might be prevented or inhibited from exhibiting them. The problem in this 

 case is analogous to the problem of determining the potencies of the blastomeres. If 

 all of the first 2, 4, or 8 blastomeres in definite animal classes can be brought to 

 develop a full embryo, these blastomeres have to be considered potentially equiva- 

 lent and their total potencies greater than those they actually display. In an anal- 

 ogous manner, if the loose mesenchyme between different organs or the mesenchymal 

 constituents of various organs which do not ordinarily develop into blood-cells can 

 be brought to do so, as is shown in this paper, the mesenchyme actually developing 

 under normal conditions into blood-cells, and that part of it which undergoes similar 

 changes under experimental conditions only, must both be considered potentially 

 equivalent and the potencies of the latter greater than those it actually displays. 

 Moreover, if mesenchymal cells, which under normal conditions invariably follow a 

 definite line of differentiation, can be brought under changed conditions to develop 

 a potency different from that manifested by them under normal conditions, these 

 mesenchymal cells must be considered polyvalent, their actual development being 

 determined by external conditions. 



There is no doubt that the physico-chemical constitution of a definite living 

 substance always plays an active part in its differentiation, and it is evident that 

 from a bird's egg nothing but the diverse tissues of a bird will develop, and from 

 a hemoblast nothing but one of the blood-cells will arise. Polyvalent structures 

 encountered in the course of embryonic development are polyvalent in a limited 

 sense only, and no longer totipotent; so a mesenchymal cell, at definite stages of 

 embryonic development still polyvalent, may have among its offspring (according 

 to environmental factors) a connective-tissue cell, a granuloblast, an erythroblast, 

 or a small lymphocyte, but certainly not a nerve-cell or a mucus gland. Never- 

 theless, the demonstration of polyvalent structures, even in a restricted sense, 

 necessarily implies the conclusion that environmental conditions may become 

 factors capable of determining the actual line of the differentiation of a polyvalent 

 structure, or, in other words, creating diversity in the living matter. 



MESENCHYME OF THE SPLEEN, OF THE ALLANTOIS, AND OF THE THYMUS. 



Descriptive investigations in embryology and pathology have furnished 

 numerous data concerning a possible polyvalency of the loose mesenchyme and of 

 the stroma of various organs in embryonic and adult life. The diffuse character 

 of the embryonic hemopoiesis is certainly suggestive, not of the dissemination of 

 specific hemopoietic primordia, but of a ubiquitous distribution of a single autoch- 

 thonous process of differentiation. Moreover, observations relating to changes of 



