4 MYELOID METAPLASIA OF THE EMBRYONIC MESENCHYME. 



capable of doing. The typical development of living matter resulting from inter- 

 action of its physico-chemical constitution and normal conditions, on account of 

 its regularity, often gives a strong impression of being merely a specific self-differ- 

 entiation, and experimental or pathological conditions only may sometimes show 

 that the regularity of typical development depends upon the regularity of the 

 environment, and that the process may vary in compliance with changes in it. As 

 a striking example of a typical process may be mentioned the development of the 

 first blastomeres in definite classes of animals. In their typical development they 

 regularly transform into definite parts of the organism, but the potencies realized 

 by them in a typical development are not the only ones belonging to them. The 

 first blastomeres in various classes of animals under experimental conditions may 

 produce not only parts of embryos but whole embryos. The typical development 

 of the first blastomeres in such eggs is limited in most cases, not by physico-chemical 

 constitution of the blastomere, but by environmental factors. The total potencies 

 of the blastomeres are greater than those which they manifest in their typical 

 development . 



In the later stages of segmentation the physico-chemical constitution of the 

 ensuing cell-groups becomes more specific and their potencies narrower. But 

 it has been already shown for some tissues that their potencies for further develop- 

 ment remain greater than those which they actually manifest. The ectoderm on 

 the surface of the whole body in amphibian larvae is found to be polyvalent, since 

 it may produce lenses at each point of its surface, if after the grafting of an optical 

 cup it enters into direct local correlation with it. W. H. Lewis says: "Any portion 

 of the inner layer of the ectoderm is capable of giving rise to a lens, when properly 

 stimulated. There is then no especial predetermined group of cells which must be 

 stimulated in order that a lens may arise." 



The important work of analyzing in higher animals the factors which determine 

 the ontogenetic processes has been inaugurated rather recently, and as yet little is 

 known as to which of the various primordia develop into their final products exclu- 

 sively on account of their physico-chemical constitution, and which of them, by 

 nature polyvalent, are compelled to do so by external factors. 



The experimental evidence found in the literature does not allow at present 

 any generalization in this respect, and it is just as incorrect to ascribe a specificity 

 to all the primordia of different organs as to declare them all totipotent, or at least 

 polyvalent. The various cell-groups have to be analyzed separately and special 

 care is required before proclaiming that an organ is definitely limited in its develop- 

 ment; for, according to Fischel, "ein negatives Versuchresultat lasst sich nicht im 

 Sinne eines Potenzmangels deuten." 



In order to find out whether local environmental factors have any determining 

 influence upon the development of definite primordia, they have frequently been 

 transplanted into other parts of the organism. Braus (1903) successfully carried 

 out similar experiments on amphibians and decided that the limb-bud of the anuran 

 larva constitutes a self-differentiating system. Harrison (1918), on the basis 



