180 H. V. WILSON AND BLACKWELL MARKHAM 



structure would have gone farther had the larva been allowed 

 to live. The yolk mass was apparently being covered in through 

 further extension over it of the blastopore lips. The mass itself 

 had made some headway toward transformation into a part of 

 an alimentary canal. The left series of myotomes would prob- 

 ably have extended itself to the tip of the tail (right tail), and 

 would have continued to grow in size. With the continued 

 dorsal extension of the coelom on the left side of the body (fig. 

 5), the conditions for the formation of a left pronephros, to match 

 that already developed on the right side, would have been more 

 nearly realized. Absorption of the left, small tail would have 

 completed the steps by which, in spite of its failure to go through 

 certain early phases of the normal cycle of changes, the embryo 

 might in the end have acquired the type structure. We hope at 

 some later date to have actual observations to report on the final 

 stages in the restoration of such asynametrically developing lar- 

 vae, if indeed they do succeed in going through the entire proc- 

 ess. It is to be expected that here, as elsewhere, in respect to 

 the actual details, individuals will vary. 



Cases essentially similar to the above have been recorded by 

 Lereboullet ('63) for the teleosts. In the embryo shown in his 

 figures 30 and 31, pi. Ill, one of the half-bodies degenerates, 

 while the other develops into a whole body. This latter comes 

 to lie in direct line with the head end. We thus get what is 

 not far from a normal embryo, a lobe-like projection from one 

 side and a split tail alone remaining to indicate the original du- 

 plicity. This conclusion, which Lereboullet draws, is made prac- 

 tically certain by the several embryos which he describes and 

 which individually illustrate different stages in the process. In- 

 deed, in the case of another embryo like his figure 31, Lereboullet 

 was able to observe from day to day the gradual degeneration of 

 one of the half-bodies. In another closely similar case, only 

 briefly described, no. 55, loc. cit., p. 224, it would seem that only 

 one blastopore lip became organized as a half-body, but Lere- 

 boullet is not very explicit here. 



The frog embryos studied by O. Hertwig ('92) do not, as he 

 l)oints out, all conform to the symmetrical type of development. 



