244 THE POPULAR SCIENCE MONTHLY 



tional significance in development, owing to the principle of relation 

 of functional area to mass. It has also another very important func- 

 tion as an isolating factor. The localizations that arise, owing to the 

 organization process of which we have spoken, are rendered relatively- 

 stable and permanent by the formation of cell-walls. Thus the ele- 

 ments of the mosaic are isolated, and each isolated part has the oppor- 

 tunity to grow into a new mosaic. Cell-division is thus an important 

 factor in progressive differentiation, not as a cause, but as a means. 



4. Environment. — Environment must be conceived in a somewhat 

 broader sense than usual in considering the individual development. 

 The developing embryo has an environment in the usual sense, con- 

 sisting of all those external conditions that surround it, some of which 

 enter into its development. But in addition to this extra-organic en- 

 vironment there is an intra-organic one; the developing embryo is 

 not merely a unit on which an extra-organic environment operates, 

 but it is a living mosaic, each element of which may conceivably enter 

 into the development of any other in the sense of being a factor in the 

 process. Each part of the embryo, therefore, has an intra-organic 

 environment consisting of all the other parts, some of which constitute 

 relatively immediate environmental factors, others relatively re- 

 mote ones. 



To illustrate: nerves arise in the embryo from certain centers and 

 grow out in the embryonic tissues, much as roots grow out in the soil; 

 the muscles arise separately and the nerves grow to them and make the 

 proper connections. Is this due to an innate tendency of each nerve 

 to grow in particular paths and branch according to definite laws, or, 

 on the other hand, is it due to a directive stimulus exerted on the 

 growing nerve by developing muscle tissue ? The answer can be given 

 only by a suitable experiment : If an abnormal innervation area were 

 brought into the field of growth of a developing nerve, would the nerve 

 entering the abnormal area follow its normal mode of branching, or 

 the one characteristic of the normal nerves of the transposed area? 

 To be specific : the bud of a leg of a tadpole that has as yet no nerves 

 may be transplanted to any region of the body (Braus and Harrison), 

 and it develops as a leg; but it receives its innervation from the nerves 

 of the region to which it has been transplanted, and the mode of 

 branching of the nerve is that of the leg nerves. We may generalize 

 this statement by saying that any nerve may be made to depart from 

 its normal mode of branching and to branch like leg nerves, by bringing 

 a leg bud into its innervation area at the time that the nerve is still 

 growing. — 



It will be seen that if this is generally true, the constancy of 

 distribution of peripheral nerves is not due to the transmission of 

 nerve-branching determinants from generation to generation, but is a 

 function of the intra-organic environment in each generation. 



