120 DEVELOPMENT OF THE FROG'S EGG [Ch. XI 



interior of the uninjured blastomere as a result of its oblique or 

 even inverted position; hence the uninjured blastomere might 

 develop differently than it would have done had it retained its 

 normal position (as in Roux's experiment). In this way we 

 might attempt to reconcile, in part, the different results of 

 Roux and Hertwig. I cannot but think, however, that the 

 main difference is due to the partial development of the injured 

 blastomere in many of Hertwig's experiments, so that cells split 

 off from the injured blastomere took part in the formation of 

 the embryo. 



In 1894 I made the following experiments to determine 

 whether one of the first two blastomeres could give rise 

 to a half or to a whole embryo, according to the condi- 

 tions of the experiment. One of the first two blastomeres 

 was killed with a hot needle in the way described by Roux 

 ('93, c).i 



In some of the eggs the black pole remained upward after 

 the operation; other eggs were rotated after the operation, 

 so that the white pole was turned upward. The eggs were 

 closely watched for several hours, in order to ascertain with 

 certainty Avhether the injured half divided or not. In those 

 cases in which this happened, the eggs in question were elimi- 

 nated from the experiment. 



The eggs were placed at first on a moistened glass plate and 

 kept for a time in a moist atmosphere, or else simply thrown 

 into water. The results seemed to be the same. When the 

 black pole of the uninjured blastomere remained up, the blas- 

 tomere developed, in all the cases observed, into a half-emhryo. 

 Conversely, those eggs in which the white pole was turned 

 upward, formed, in most cases, wliole embryos of lialf-size. In 

 the latter case the cleavage was modified in consequence of the 

 reversed position of the egg. The upturned white hemisphere 

 produced smaller cells than the lower black hemisphere, point- 

 ing unmistakably to a rotation of the fluid contents of the 

 blastomere. 



1 The needle was heated each time before piercing an egg. This made a 

 greater injury to the blastomere much more certain. On the other hand, it 

 lowered the percentage of embryos obtained, because in many cases the other 

 blastomere was probably injured also by the heat. 



