237 



of Eycleshymer ('98), all pointing to the conclusion that both dorsal 

 and ventral lips move. Finally the weakness of Morgan's conclusion 

 is demonstrated in the (rare) cases where the ovate for some reason 

 is not carried along with the ventral lip (Exp. 7). 



Experiment 10. The position in which the neural plate ap- 

 pears, in eggs which owing to adherence between the glass and the 

 ovate, have been rotated out of their normal position, is interesting. 

 Seven of the eggs described under Exp. 9, in which the blastopore 

 at 1 A.M. occupied a very excentric position (Fig. 16^''), were kept 

 for observations on this point. — In six of these eggs, rotation took 

 l)lace during the night through or nearly through 180^ in a trans- 

 verse vertical plane, so as to give the next morning at 10 A.M. 

 views of the upper surface such as are shown in Fig. 16^",-''. The 

 explanation of this peculiar rotation is not far to seek. In the case 

 of an egg, in which the extraovate has caused the blastopore to lie 

 excentrically on the under surface, a rotation in the longitudinal 

 vertical plane (usual plane of the 90" rotation) of nearly 180*^ would 

 be necessary in order to bring the blastopore into its final position 

 (at one end of the horizontal axis, with the dorsal lip uppermost). 

 But the extraovate forms an obstacle to rotation in the longitudinal 

 vertical plane. The under surface of the egg must however owing 

 to the change in the relative specific gravities of its internal parts, 

 become the upper surface ; and since rotation through the longitudinal 

 vertical plane is prevented, a rotation through the transverse vertical 

 plane occurs. — In the case of the seventh egg, the egg was held 

 so firmly in its abnormal position that this rotation did not occur, 

 and the next morning at 10 A.M., the neural folds were found de- 

 veloped on the under surface of the egg, Fig. 16 -^ mirror picture. 

 Here it would seem that the abnormal position of the neural folds 

 has been brought about by a simple mechanical cause, which has first 

 placed the whole egg in an abnormal position, and the succeeded in 

 keeping it there. 



The appearance of the neural folds on the under side of the egg 

 in the case just described may have some bearing on the compression 

 experiments of Pflüger, Roux and Born, which have been thought 

 to prove that the neural folds develop over the white hemisphere. 

 Born's experiments ('93) are those in which the conditions are most 

 precisely given, although the point is a minor one in Born's paper, 

 the author expressly mentioning that he has paid no especial attention 

 to the migration of the blastopore. Eggs were compressed with the 

 black pole up between glass plates, and fertilized. With the pecu- 



