456 MORGAN. [Vol. X. 



when the dorsal lip is destroyed, and its backward growth pre- 

 vented, nevertheless, the medullary folds come up from the 

 sides to form in the middle line, posterior to the point of 

 injury, the central nervous system and notochord of the 

 embryo. There is often left a large yolk-plug, either just 

 behind the anterior cross-commissures, or more posteriorly, in 

 the middle of the medullary plate. 



It is possible that when the dorsal lip of the blastopore 

 was injured in the middle line, its material has divided and 

 extended backwards on each side along the black-white line. 



In order to test this I operated on a number of frog embryos, 

 destroying not only the dorsal lip in the middle line, but also 

 making a series of injuries on each side of the dorsal lip as 

 well. By this means the possibility of a division of the central 

 mass into a right and left half would seem to have been pre- 

 vented. Under these circumstances the blastopore closes 

 laterally and behind. Serial sections were made to see if 

 by this closure the normal structures were formed, or if the 

 closure had nothing to do with the normal process. The sec- 

 tions showed beyond a doubt that the right and left sides of 

 the blastopore had brought up to the median line the right and 

 left sides of the embryo. 



These and other experiments that need not be described 

 here have convinced me that the material for the embryo of 

 the frog is laid down in a ring of cells around the black-white 

 line, and that this ring approaches the mid-line during the 

 closure of the blastopore, i.e., the material reaches the mid-line 

 from before backwards. From the evidence gained by these 

 experiments, we are forced to conclude that in those cases 

 where the embryo formed as a ring around a large yolk exposure, 

 the greater elongation of the sides of the embryo is due to 

 the material differentiating in situ. The material for the 

 nervous system and notochord must form a ring quite far 

 up on the sides of the blastula. The ring is not, however, as 

 a large series of new preparations show me, placed so far up 

 as the equator of the ^g^. 



The bifurcation in the spina bifida embryos is caused, then, 

 as Roux and Hertwig state, by a failure of the two sides to 



