384 



THE AMERICAN NATURALIST [Vol. XLIII 



translator, in presenting this edition without amendment of 

 any kind. 



George H. Shull. 



EMBRYOLOGY 

 On the Totipotence of the First Two Blastomeres of the Frog's 



Egg — It was found by Roux that if one of the first two blasto- 

 meres of the frog's egg is injured with a hot needle, the unin- 

 jured blastomere will develop into a half embryo. Morgan 

 found, however, that if the egg were kept in an inverted position 

 after the injurs' of one blastomere, the other would develop into 

 a whole embryo. He attributes this difference to the rearrange- 

 ment of the "mosaic" structure of the egg by flowing of sub- 

 stance, of different specific gravity (Born). However, he sug- 

 gested to me that the half embryo obtained by Roux might be 

 due to the presence of the injured blastomere. In Triton, when 

 the first two blastomeres are separated, each gives rise to a whole 

 embryo (in case the first cleavage plane would have become the 

 median plane). I have been trying various methods for re- 

 moving one of the first two blastomeres of the frog's egg, and 

 succeeded in getting a small per cent, of the remaining blasto- 

 meres (of ChorophUus trisi ri<rfus) to develop. The puncture of 

 the egg membrane caused it to shrink down on the remaining 

 blastomere and left an opening for bacteria. Although there 

 was a great mortality in the operated eggs, quite a number of 

 them gastrulated (as wholes), and several of them reached the 

 tadpole stage. The pressure of the egg membrane seemed to 

 hinder their further development and none of them hatched, 

 although one lived until I fixed it for sectioning, after the con- 

 trol eggs had hatched. In no case was a half gastrula found, 

 and as this is the earliest stage at which the bilateral symmetry 

 is very pronounced, all of the embryos were wholes as far as 

 could be observed. Probably all operated eggs in which the 

 median plane would not have coincided with the first cleavage 

 plane died before gastrulation. 



J. F. McClendon. 



University of Missouri, April 8, 1909. 



