CELLULAR DIFFERENTIATION AND INTERNAL ENVIRONMENT 



89 



The induction of the crystalline lens by 

 the optic vesicle is another notable ex- 

 ample of dependent differentiation. This 

 case is complicated, however, by the fact 

 that in some species of amphibia and fish 

 the lens is capable of self-differentiation 

 without the influence emanating from the 

 optic cup. The natural conclusion to draw 

 from these facts is that the lens rudiment 

 itself includes the factors necessary to form 

 a lens and that the optic cup is needed in 

 some cases to set off the chain of reactions 

 that produce it out of its ectodermal rudi- 

 ment, while in other cases this specific 

 action is not required. 



The amphibian limb is an organ that is 

 localized at least as far back as the gastrula 

 or perhaps even earlier, as shown by both 

 transplantation and defect experiments 

 (Detwiler 1929; Suzuki 1928). However, 

 it was found by Balinsky (1925, 1931, 

 1933), and later confirmed by others, that 

 by implanting in the flank of an embryo 

 such organ rudiments as ear placodes, nasal 

 placodes, pieces of brain or, in some cases, 

 even small pieces of celloidin, more or less 

 perfect limbs could be induced Here, 

 cells that would ordinarily form axial 

 musculature and mesenchyme on the flank 

 of the organism, are now activated to form 

 an appendage with all of the complex ar- 

 rangements in muscle, cartilage, bone, etc., 

 that constitute a limb and are quite foreign 

 to that region. No satisfactory explanation 

 of this has been given. 



All the reactions described in the fore- 

 going, with the possible exception of the 

 inhibition of the spread of the apical tuft 

 in the sea urchin, apparently take place 

 between cells that are in contact. When 

 neither the nervous system nor the circula- 

 tion is developed, it is indeed difficult to 

 imagine a reaction at a distance. Proto- 

 plasmic transmission is a slow process, and 

 there is in fact almost no evidence of re- 

 action between cells that are far removed 

 from one another in the early stages of 

 embryonic development. The clearest case 

 is a phenomenon observed by T witty (1937) 

 in experiments in which small grafts (eyes) 

 from Triturus torosus were implanted in 



amphibian embryos of other species. Such 

 grafts completely paralyze the host embryo 

 for a time, and this action takes effect be- 

 fore the circulation begins. It can only 

 be explained by the diffusion of the toxic 

 substance through the tissues of the host. 



In growth, as well as in differentiation, 

 cells and cell groups are dependent upon 

 one another. The proportions of an organ- 

 ism are acquired through the maintenance 

 of the normal relative growth of its parts. 

 Delicate adjustments are here necessary, 

 but they are little understood, though 

 known to be governed by both genetic and 

 environmental factors. Rates of growth 

 are specific, and each organ or part has its 

 characteristic rate relative to that of the 

 whole. Relative growth rates are suscep- 

 tible to external and internal environmental 

 influences, though in a much less degree 

 than the general growth of the whole 

 organism. 



The specificity of growth rates is strik- 

 ingly shown by grafting organs, such as 

 limbs and eyes, between different species 

 (e.g., Amhly stoma tigrinum and A. puncta- 

 tum) which grow at different rates. 

 Grafted organs retain their characteristic 

 rates and the result is an organism, often 

 grotesque in appearance, with a limb or an 

 eye that is either much too large or too 

 small for it. Such cases show that although 

 subjected to the circulating medium of the 

 host, congenital specific factors control the 

 relative growth rate of the graft (Harrison 

 1924, 1935). 



Germ layer chimaeras in the case of 

 limbs, i.e., individuals in which either the 

 ectoderm or the mesoderm is transplanted 

 alone, show that the relative growth rate 

 of the appendage as a whole is governed 

 almost entirely by that of the mesoderm, 

 the ectoderm having very little effect upon 

 it. In the eye, however, both optic cup and 

 lens are concerned in maintaining the rela- 

 tive growth rate of the whole organ. Each 

 of these components stimulates or retards 

 the growth of the other, according to rela- 

 tive growth rates in the two species, so that 

 an eye of approximately correct internal 

 proportions, though still out of scale with 



