INDIVIDUAL DEVELOPMENT 245 



The case of the determination of nerve-branching by intra-organic 

 relations does not by any means stand alone. The same principle 

 undoubtedly holds for the development of the blood-vessels, which 

 grow along paths determined by the arrangement of organs and tissues 

 and not according to a predetermined law given in the blood-vessels 

 themselves. Color patterns have been shown in some cases to be 

 determined by intra-organic variable relations, as in Loeb's experiments 

 on the determination of the color pattern of the yolk-sac of a fish, which 

 he demonstrated to be due to the positive attraction of the circulating 

 blood for migratory cells that bear pigment. The development of any 

 color pattern was therefore dependent upon blood-circulation, and the 

 form of the pattern upon the pattern of the blood-vessels. The 

 primordia of the eye or the ear transplanted to strange locations in the 

 embryo induce formations in surrounding tissues that are strange to 

 them and characteristic of the normal eye and ear environment. The 

 origin and growth of motor nerve cells has been shown in my laboratory 

 by Miss Shorey to be dependent in the chick on normal muscle develop- 

 ment; so that the anatomy of the central nervous system, no less than 

 the peripheral system, is dependent to some extent on the environment. 

 Eegeneration of lost parts is dependent for its completion to some degree 

 on innervation, and the normal development of muscle tissue beyond a 

 certain stage is likewise so dependent. These examples might be in- 

 creased by others, which, taken together, would show that an immense 

 part of what we call inheritance is inheritance of environment only, that 

 is, repetition of similar developmental processes under similar condi- 

 tions. The bearing of all this on the doctrine of determinants, that 

 characters of the adult are represented by germs of a lesser order in the 

 germ of the entire organism, is obvious. 



Many of the problems of heredity, so-called, are not capable at 

 present of such resolution. We may note some instances of this kind 

 and then attempt to analyze the whole matter briefly. The example 

 cited of transplantation of a leg-bud is of this kind: the transplanted 

 leg-bud does not develop into an arm if it be transplanted to the region 

 of the arm, but into a right or left leg, as the case may be, and this is 

 true no matter how early the stage at which the transplantation may 

 be made. It is not possible to change the specificity of such a pri- 

 mordium by any means yet employed. Moreover, there are many other 

 experiments which show that the primordia of a great many structures 

 are definitely specified even before they can be detected by any method 

 of pure observation. Thus if a portion of the medullary plate of a 

 frog embryo be cut out so as to include in the cut part the region that 

 would form an eye in the course of time, and if then this piece be re- 

 placed inverted, it is found that the subsequent development of this area 

 is inverted, not restored to the normal, although no trace of organs 



