AMPHIBIA, REPTILES, OTHER CHORDATES 93 



case of physical isolation of the equivalent halves of a 

 single embryonic axis closely parallels what we believe 

 occurs through physiological isolation in other groups. 

 Inasmuch as this work is to be discussed in a subsequent 

 chapter dealing with the matter of reversed S}TQmetry, 

 we shall omit further details in this place. 



Bellamy (19 18), in his elaborate series of experiments 

 dealing with the modification and control of development 

 in the frog, obtained one double-headed individual and 

 not a few with single head and more or less completely 

 double bodies and tails. Some of the individuals were 

 double as far forward as the forebrain; others were double 

 only in the tail region. This writer was, at the time of 

 his experiments, not much interested in twinning and did 

 not discuss the matter at any length in his paper. He 

 merely says that ^^ spina bifida of all degrees is of course 

 common under conditions that inhibit development, 

 and result primarily from inhibition of the dorsal lip 

 region" of the blastopore. Bellamy thus takes a posi- 

 tion, in harmony with my own, that these duplicities 

 which he has merely referred to as '^ spina bifida'^ are 

 cases of bilateral twinning and are due to the inhibition 

 of a particularly susceptible region of the embryonic 

 axis, the dorsal lip of the blastopore. If this region of 

 growth is inhibited and lateral regions continue to grow, 

 twinning is inevitable. It may be said that the dorsal 

 lip region is probably a near equivalent of the region 

 of the fish embryonic shield called by Kopsch the Knopf, 

 which is the primordium of the tail-bud. We have 

 already shown that the tail-bud region is a secondarily 

 acquired point of high metaboHc rate and very active 

 growth. Such a region might readily be so inhibited as 



