KEGULATION IN ANURAN EMBRYOS 



173 



ing at once into the tissue of the blastopore hp, on the left thin- 

 ning out at a little distance from the notochord hito a sheet, 

 which farther outwards is not distinguishable from the dorso- 

 lateral wall of the archenteron. Peristomal mesoderm, y.m., has 

 begun to differentiate on the other side of the embryo extending 

 ventrally from the right blastopore lip, r.h.l. Otherwise this lij:*, 

 the right one, shows no differentiation. 



^n 



] 2 



Fig. 1 Bufo embryo. Dorsal view, -pn, posterior end of neural plate; Ih, 

 tail buds. X IS. 



Fig. 2 Transverse section of embryo shown in figure 1, through the blasto- 

 pore, arch, archenteron; de, dorsal entoderm; Ibl, left blastopore lip; ImJ, left 

 medullary fold; nc.m,, gastral mesoderm; v.m, peristomal mesoderm; rhl, right 

 blastopore lip; rtnj, right medullary fold. X 40. 



It is clear that in this embryo a full set, not a half set, of axial 

 structures had begun to develop on the left side of the blastopore. 

 In this differentiation of the axial body, the right lip had no share. 



CHOROPHILUS LARVA WITH ASYMMETRICAL REGULATION 



The frog larva next to be described (figs. 3, 4, and 5) represents 

 fairly well a later stage of the embryo shown in figure 1, suppos- 

 ing that the asymmetrical regiilation begun in the latter had 

 gone on. This embryo appeared in a batch of normally develoj)- 



