REGULATION IN ANURAN EMBRYOS 179 



The other, originally left, tail has a distinctly developed series 

 of small myotomes on one side, the side turned away from the 

 yolk mass (originally the left side). This extends from about 

 the tip of the tail to its base and forward as a very small stripe 

 for some distance, close to the morphological left lip of the blasto- 

 poiie, fading away anteriorly. It is evident, then, that the left 

 lip of the blastopore had made steps toward organization. But 

 the'regulatory processes going on along the other lip made these 

 steps futile. 



Pronephros. The right pronephros, figure 5, r.pr., is well de- 

 veloped and the right Wolffian duct extends backward through 

 the trunk. What appears to be the left Wolffian duct is present 

 near the left blastopore edge, about the middle region of the body, 

 extending through a number of sections. The anterior end of 

 this tube could not be traced. Thus whatever steps have been 

 taken to form a left pronephros have not gone beyond the merest 

 start. 



In the embryo just described the axial organs (neural tube, 

 notochord, myotomes) have not, it is clear, been built up by the 

 fusion in the middorsal line of two halves. Instead, an asym- 

 metrical method has been followed, the fundamental feature in 

 which is the utilization of only one blastopore lip. The steps 

 in this process are briefly as follows: 1) The exposed yolk area is 

 shifted over toward the left side. 2) The right blastopore lip 

 and the right tail bud are thus brought in line with the median 

 axis of the head end. 3) The right lip organizes a backward con- 

 tinuation of neural tube, notochord, and the right series of myo- 

 tomes, all continued into the right tail bud, which grows much 

 larger than the left one. 4) After having established axial or- 

 gans in the trunk region, the right blastopore lip grows for some 

 little distance over the exposed yolk, thus coming to lie to the 

 left of the median body ]ilane. .V series of myotomes is then or- 

 ganized along the left side of the notochord. 5) The left lip of 

 the blastopore and the left tail bud make no more than the first 

 steps toward organization. 



The behavior and the anatomy of the larva at the time of 

 preservation indicated that the process of establishing the normal 



