October 6, 1916] 



SCIENCE 



507 



In my attempt to gain some insight into 

 the manner in which these changes might be 

 effected in the absence of coercion from with- 

 out, I determined, at constant magnification, 

 the areas of comparable sections during the 

 process of involution. The marked relative 

 increases shown in Table III. were found. 7 



TABLE III 



Relative Areas of Comparable Sections 



This increase in area indicates growth in 

 volume, and can be the result only of enlarge- 

 ment on the part of the individual cells con- 

 stituting the nervous system. The immediate 

 problem is obvious. 



In Table IV. are given the water-content as 

 well as the distribution of water in the larvse 

 of Rana pipiens and Ambly stoma punctatum, 

 four to five days after fertilization. 



Since the period of differentiation under 

 discussion has been completed at the stage of 

 development considered in the table, and since 

 this differentiation includes folding, and fold- 

 ing is associated with enlargement, it follows 

 that the differential absorption of water by the 

 nervous system probably took place during the 

 process of involution. As the results show, 

 the water-content rises to a point practically 

 identical with the figure 80.5 per cent, given 

 by Donaldson for the cord of the adult 

 B. pipiens.* 



7 For details concerning the distribution of this 

 increase within the sections themselves, see Glaser, 

 loc. cit., pp. 530-533. 



8 Donaldson, Henry H., "Further Observations 

 on the Nervous System of the American Leopard 

 Frog, etc.," Jour. Comp. Neurol., Vol. 20. Also 

 earlier papers. 



Water Content and Distribution of Water in 



Embryos of Sana pipiens and Amblystoma 



punctatum Four to Five Days after 



Fertilization 



But this absorption of water can only ac- 

 count for the enlargement of the nervous sys- 

 tem, not at all for its folding. 



To explain this in complete harmony with 

 all the known facts, only one assumption is 

 necessary. The neural plate is exposed to an 

 external environment, whose constancy, within 

 the limits under which normal development 

 takes place at all, is very high. Laterally each 

 cell of the plate is bounded by a chemical sys- 

 tem fundamentally like itself. Disturbances 

 of equilibrium on any one of these surfaces 

 are relatively unlikely. However, on its under 

 side, the plate is subjected to a constant change 

 of conditions due to the multitude of processes 

 going on within the rest of the embryo. To 

 mention only one factor, there is a distinct in- 

 crease in the acidity of the internal medium. 



On this basis we may interpret the absorp- 



