231 



as far as the frog embryo is concerned falls to the ground. Hert- 

 wig's teratological evidence (spina bifida cases— so convincing to 

 many minds) is purely circumstantial, and by itself is as well ex- 

 plained on the hypothesis of a rupture, which we must suppose to 

 progress gradually away from the dorsal lip during the motion of 

 that lip towards the lower pole, as it is by the concrescence theory. 

 The very instructive counterpart of the spina bifida embryo, it 

 seems to me, is to be found in embryos such as Hertwig ('93, '95) 

 has reared in werk salt solution. In the latter it is the ventral wall 

 which is ruptured instead of the dorsal, the rupture passing poster- 

 iorly into the true blastopore. 



1. Ovate produced in the centre of the black hemi- 

 s p h e r e. 



Experiment 1. On March 19, at 1.45 P.M., 13 Amblystoma 

 eggs in a late stage of segmentation were pierced as nearly as pos- 

 sible in the centre of the black hemisphere — the position of the 

 ovate is shown in Fig. 16', a side view of the egg. The eggs were 



placed on a mirror and covered with water. The ovates in the natural 

 position of the egg were not easy to see. Careful examination showed 

 however that in only three out of the thirteen eggs, was the ovate 

 perceptibly out of the centre of the upper hemisphere — and in these 

 three eggs the distance from the centre was slight. — The blastopore 

 came in at 10 P.M., appearing through a lens as an actual cleft in 

 the white hemisphere. The next morning at 11 A.M. it was circular, 



