184 THE ANCESTRY OF VERTEBRATES 



more than 180°. The place where the dorsal blastopore 

 lip first appears is in all three cases nearly the same, 

 being situated a little beneath the equator of the egg. The same 

 holds good for the place where the crescent-shaped blastopore 

 border closes to a ring, i. e. where the ventral lip first 

 appears. In all three cases this is close to the vegetative pole. 



In Rana esculenta, however, we see the ventral lip mov- 

 ing forward more actively than in Rana fusca, which 

 causes the blastopore to close at a place situated more to 

 the dorsal side than in Rana jusca 1 must rectify here 

 a statement made with some reserve in a former paper 

 (1916). I gave a figure (fig. 8, p. 8) in which tlie border 

 of the blastopore, at the moment it has just closed to a 

 ring, was provisionnally indicated with a dotted line, since 

 I had not observed it in the egg figured there but had 

 transferred it into this figure from other eggs. Afterwards, 

 however, repeating my experiments I could state that the 

 latter eggs were such that had been developing abnor- 

 mally and showed a tendency to the "spina bifida" phe- 

 nomenon, in normal eggs the blastopore has never so 

 large a diameter which seems to be caused by a bulging 

 out of the yolk-plug and a temporary forward instead of 

 backward movement of the dorsal blastopore lip, as men- 

 tioned above. Thus this dotted line must be removed from the 

 figure. Further experiments have also given me the impres- 

 sion that the movement of the ventral lip has been shown 

 a little too large in this figure and that in reality it does 

 not move faster than the dorsal lip. Also the dorsal movement 

 of the whole white area, to which I thought it inevitable 

 to conclude from the observed facts, was not confirmed by 

 the observation of eggs marked at both the vegetative and 

 the animal pole. As far as 1 could state both marks remained 

 exactly opposite each other during the cleavage and the 

 gastrulation, just as in Rana fusca. 



Thus the difference between Rana fusca and Rana 

 esculenta appears to be much less than I first concluded 

 from experiments on eggs, part of which afterwards proved 

 to develop abnormally. In Rana esculenta the ventral lip 

 moves only a little more actively than in Rana fusca, and 

 also in longitudinal sections the ventral lip appears to be 

 a little more strongly developed than in the latter form. 



The reverse case is found in Amblystonia. Here the 

 dorsal blastopore lip, after its first appearance a little 



