506 



SCIENCE 



[N. S. Vol. XLIV. No. 1136 



therefore, a neural plate folds in spite of an 

 increase in the number of its constituent cells. 



.3 <3 



Fig. 1. Diagrammatic representation of a neu- 

 ral plate A B C D, conceived of, for the sake of 

 simplicity, as entirely flat and made up of one 

 layer of rectangular cells. The lower half of the 

 figure shows the same plate symmetrically folded, 

 its upper and under sides having become the out- 

 lines of two concentric circles. With the cells 

 constant in number and position, the line A B is 

 now necessarily shorter than the line C D. 



Pig. 2. Two sections through the embryonic 

 nervous system of Cryptobranchus allegheniensis, 

 showing the nuclear distribution in Stages I. and 

 II. The sections are from the same series and 

 regions as those dealt with in the tables but eon- 

 tain for Stage I., six, and for Stage II., one 

 nucleus more than the maximal number recorded 

 in Table I. In the unfolded plate there are in the 

 present case, 78 nuclei, of which 47 are in the 

 upper half above the dotted line, and 31 in the 

 lower; in the half -folded plate, there are 75 

 nuclei, 21 in the upper zone, and 54 in the lower. 

 Nuclei which happen to fall on the line separating 

 the two zones are ascribed to the one into which 

 the greater portion of their mass projects. 



The only remaining way in which a neural 

 plate can fold itself is by a rearrangement of 

 materials present at the beginning. In this 

 connection the most patent fact, emphasized 

 long ago by Ehumbler 5 and Conklin 6 in their 

 studies of invaginate gastrulation, is a change 

 in the shape of the cells whose sectional out- 

 lines alter from the rectangular form to that 

 of a trapezium (Fig. 1). This geometrical 

 transformation, which might be forced upon 

 the cells from without, necessarily has the 

 same result when autonomously produced, for 

 it involves lengthening of one surface, short- 

 ening of the other, and a redistribution of the 

 cell-contents. The extent of the latter, as indi- 

 cated by the migration of nuclei from the side 

 becoming concave to that becoming convex, is 

 clearly shown for two sections in Pig. 2, and 

 for a series, in Table II. 



Distribution of Nuclei in Upper and Lower and 

 Inner and Outer Zones 



According to this, the distribution of the 

 nuclei is not only completely reversed during 

 folding, but the final relation between the 

 number in what become the inner and outer 

 zones, respectively, of the definitive tube, is as 

 2 to 1. Provided only that the nervous system 

 is, by its structure and relations, incapable of 

 indefinite expansion, these changes are all that 

 are required to bring about the folding. 



5 ' ' Zur Mechanih des Gastrulationsvorganges, ' ' 

 Arch. f. Entwicldungsmech., Bd. 14. 



6 ' ' Mosaic Development in Ascidian Eggs, ' ' 

 Jour. Exp. Zool., Vol. 2, p. 163. 



