216 HUBERT DANA GOODALE 



is deflected by a rotation of the contents of the egg away from the 

 plane occupied by the penetration path. Since the former deter- 

 mines the direction of the plane of cell division it explains, formally 

 at least, those cases in which the first plane of cleavage deviates 

 from the median plane of the embryo. In Spelerpes, owing to 

 the relative large perivitelline space present when the egg is 

 laid and the long time elapsing before the first division, the 

 egg would seem to be under normal conditions in Roux's sense, 

 from the very start. Hence I believe no objections can be raised 

 on this ground to the use of eggs collected in the field. 



The lack of agreement between the first plane of cleavage and 

 the median plane of the embryo has an important bearing on 

 the numerous experiments made to test the potentiality of the 

 blastomeres. If one of the first two blastomeres of the frog be 

 killed, a half embryo may develop from the living one. Morgan 

 ('95) by first killing one of the blastomeres and then inverting the 

 egg, was able, often, to produce a whole instead of a half embryo. 

 Schultze ('94) followed by Wetzel ('96), was able to produce 

 double embryos by inverting the egg after the first cleavage was 

 completed. These results, considered in the light of the failure 

 of the median plane of the embryo to coincide with the first 

 plane of cleavage, merely show that, if a lateral half of the material 

 destined to the formation of the embryo, be killed (or if the two 

 halves be separated, as in the recent experiments of McClendon, 

 '09) it is able, under suitable conditions, to rearrange itself to 

 form a whole embryo, but it does not prove that the blastomeres 

 are necessarily either equipotent or totipotent. Spemann ('02), 

 by ligaturing the egg of Triton in one of the first planes of cleav- 

 age,'-* finds that while the first two blastomeres may produce two 

 embryos if the first plane of cleavage and the median plane of 

 the embryo coincide, that when these two planes lie at right angles 

 with each other, one blastomere develops into an embryo and the 

 other into a structure comparable with the ventral half of an em- 

 bryo. Spemann's results prove, I believe, that the first two blas- 

 tomeres are not equipotent unless there is a coincidence between 



' He is confirmed by Hrdlicka's results, though not so stated bj- llrdlicka. 



