544 ROSS G. HARRISON 



spaces between the main organ systems of the body which are 

 aheady laid down. In this process the surface of structures 

 such as the medullary cord, notochord, almientary canal, muscle 

 plates and the inner surface of the epidermis would serve as a 

 solid base upon which the cells might creep. It is found that 

 sooner or later all of these surfaces become ensheathed or lined 

 by connective tissue cells. In the encystment of foreign bodies 

 within the organism a similar phenomenon is observed. 



With regard to the movements of the growing nerve fiber the 

 e\'idence, as pointed out above, is not quite so varied, but it 

 is sufficient to warrant the conclusion that also this protoplasm 

 is stereotropic. No free outgrowth of nerves in a fluid medium 

 has ever been observed, while such solids as the fibrin clot and 

 smooth glass surfaces serve readily to support them, as do the 

 surfaces of the larger cell masses and the interstitial protoplasmic 

 network inside the embryo. 



Most of these points were discussed some years ago by Herbst,^^ 

 who, however, did not claim to have reached a definite conclusion 

 as to the exact kind of reaction involved. The experiments here 

 described do not of course settle the question either, but since 

 it has been shown that most embryonic cells are stereotropic, 

 and that such arrangements as they assume in the embryo may 

 often be induced under cultural conditions by reactions to solids, 

 there is a presmnption in favor of the view that this type of re- 

 action is a potent factor in normal development also. Inferences 

 as to what goes on in the embryo which are not based upon exact 

 information regarding the physiological properties of the tissue 

 elements are likely to prove erroneous. On the other hand, 

 if we know the actual properties of individual cells in detail, it 

 will be possible to form, on the basis of the observation of normal 

 development, an accurate conception of the influences actually 

 at work in shaping the embryonic body. 



33 Biol. Centralbl., Bd. 14, pp. 746. 



