GROWTH, METAMORPHOSIS AND DEVELOPMENT 261 



advance of the silver nitrate. What is shown here in the case of the 

 silver chlorid holds for every other precipitate, and for every osmotic 

 disturbance, provided only that dif- 

 fusible substances are present in a 



jelly. 



If such disturbances (precipi- 

 tates, membranes) occur simultane- 

 ously in various places, it is possible 

 for the most complicated figures to FlG ' 46 /. Apparent chemical attrac- 



tion. (R. Liesegang.) 

 form. 



In addition, we must consider the changes which occur in the ordi- 

 nary course of swelling and shrinking of the colloidal material. 



As far as I know, it has hitherto not been poss$>le to explain 

 such a phenomenon in vivo. I wish to refer to only one analogy: 

 C. U. ARIENS-KAPPERS* described as neurobiotaxis a phenomenon of 

 nerve fibers: if two nerve cells, a certain distance apart, are injured 

 simultaneously or in close succession, the growth of the chief dendron 

 of both injured ganglion cells occurs in the direction of the other 

 stimulated or injured cell. We thus have a growth towards each 

 other, analogous to the apparent chemical attraction just described. 



[C. A. ELSBERG concludes from his experiments that hyperneuroti- 

 zation of a normal muscle is impossible. A normal muscle cannot be 

 made to take on additional nerve supply. The implanted nerve can- 

 not make neuro-motor connections. If the muscle is permanently sep- 

 arated from its original nerve, the implanted nerve will then establish 

 such connections. Science N. S., Vol. XLV, p. 319 et. seq. Tr.] 



Naturally, cells mutually modify each other's shape. A structure 

 which would develop spherically if uninfluenced, under the pressure of 

 neighboring cells acquires a reticulated, fibrous or pavement shape. 



Layered Structures. 



We saw that if two solutions which form a precipitate meet in a 

 jelly, a precipitation membrane develops at the point of contact. 

 Provided this is sufficiently permeable and one solution has a higher 

 osmotic pressure, the membrane continues to grow uninterruptedly, 

 becoming constantly thicker until the osmotic pressure is the same 

 on both sides. In 1898, R. E. LiESEGANG* 1 published an observation 

 which does not accord with the continuous growth mentioned above. 



If, for instance, ammonium bichromate is dissolved in melted 

 gelatin, which is then solidified in shallow dishes, and upon it a drop 

 of silver nitrate is placed, then there does not develop upon diffusion a 

 constantly thicker precipitation membrane of silver chromate but con- 

 centric rings called LIESEGANG'S rings (see Plate II, Fig. 50). The 



