78 



DEVELOPMEXT OF THE FROG'S EGG [Cn. VII 



the blastopore is injured with a needle at the moment of its 

 appearance, or if the yolk-mass in front of the dorsal lip is 

 injured so that the yolk protrudes from the general rounded 

 surface of the egg. The blastopore is thus prevented from 

 extending backward, and its material differentiates, in situ, 

 along the equatorial line. The lateral lips tend to approach 

 the middle line and to fuse, but the medullary folds may 

 appear before the fusion has taken place. There is thus pro- 



FIG. 29. Cross (A, B) and longitudinal (C) sections through an embryo with spina 

 bifida. (After Hertwig.) M. Half medullary plate. N. Half uotochord. Y. Yolk. 



duced an embryo with an exposure of yolk in the mid-dorsal 

 line. The exposure is more or less extensive, according to the 

 extent of fusion anteriorly of the blastopore, and to the extent 

 of fusion forwards of the lateral and ventral lips. 



These embryos with spina bifida show that the material for 

 the mid-dorsal surface of the embryos appears first as a ring 

 around the equator of the egg or a little below the equator. 

 If this material is prevented from reaching the mid-dorsal 

 surface, it differentiates in situ. Hence the production of a 

 ring-like medullary plate and a double notochord. 



