386 THE EVOLUTION THEORY 



in, so to speak, supplying the soil in which ultimately its own deter- 

 minants will be sown from the nucleus, and whose influence modifies 

 these last according to its quality. We might therefore say that 

 every part is determined by all the determinants of its cell-ancestors. 



If there be urged against the doctrine of determinants the 

 undoubted fact of the dependence of individual development on external 

 conditions, or the capacity that organisms have of functional adaptation, 

 or especially the power that some parts of the organism have of taking 

 a different form in response to different stimuli, I can only say that 

 I see no reason why certain cells and masses of cells should not be 

 adapted from the first for responding differently to different stimuli. 



Therefore I see no contradiction of the determinant theory when, 

 for instance, among the higher vertebrates, the cells of the connective 

 tissue exhibit a great diversity of form, becoming a loose ' filling ' 

 connective tissue in one place, a tense fascia, ligament, or tendon tissue 

 in another, according as they are subjected to slight pressure on all 

 sides or to stronger pressure on one side. I see no difficulty in the 

 fact that this connective tissue forms in one case bone-tissue with the 

 most accurate adaptation of its microscopic structure to the conditions 

 of stress and pressure which affect the relevant spot, or in another 

 case cartilaginous tissue, when the cells are exposed to varying 

 pressure (as on the surface of joints), or even that it gives rise to 

 blood-vessels when the pressure of the circulating blood and the 

 tension of the surrounding tissues supply the necessary stimulus. 

 It is easy to see how important, indeed how necessary, the many- 

 sidedness of these cells is for the organism, even leaving out of 

 account such violent interference as the breaking of a bone, the 

 irregular healing of broken ends of bones, new joint formation, and 

 so on, and thinking only of the normal phenomena of growth. While 

 the bone grows it is continually breaking up in the inside and 

 forming anew on the surface, and this occurs through the power of 

 the connective tissue-cells to form different tissues under different 

 influences or stimuli. 



We must therefore assume that there are side by side in the 

 connective cells of higher vertebrates determinants of bone, of 

 cartilage, of connective tissue in the narrower sense, and of blood- 

 vessels, and that one or other of these is liberated to activity 

 according to the stimulus affecting it. Phenomena occur also in the 

 development of lower animals which lead us to the same assumption. 



Among these is the remarkable behaviour of the primary 

 mesoderm-cells in the young embryo (gastrula) of the Echinoderms 

 {Fig. 92). At the point where the primitive gut or archenteron 



