CLEAVAGE AND DIFFERENTIATION 93 



to purely mechanical reasons ; and the result is the formation of a 

 double monster. This means that each blastomere at the 2-cell 

 stage of the frog is capable of giving rise to more than it would 

 produce in normal development, and therefore the various regions 

 of the egg cannot all be determined at this stage. ^ 



It is clear, therefore, that it is not the mere presence of the injured 

 blastomere, when the latter is pricked with a hot needle at the 2-cell 

 stage, which prevents the other blastomere from developing into 

 a complete embryo. This is still more evident from the experiment 

 in which one blastomere of the 2-cell stage is injured as before, and 

 then the embryo, injured and uninjured blastomeres together, is 

 inverted and maintained in that position. The uninjured blastomere 

 will then develop into a more or less complete embryo.'^ The in- 

 version results in a streaming of the contents of the uninjured 

 blastomere so that the yolk again becomes undermost, and it is to 

 this rearrangement that the power of developing into a whole 

 emibryo on the part of a single blastomere must be ascribed. It 

 must be because there is no such rearrangement in the case where 

 a blastomere is injured and the embryo is not inverted, that the 

 uninjured blastomere in such an experiment develops into a half. 

 The presence of the injured blastomere necessitates the retention 

 of the hemispherical shape on the part of the uninjured blastomere, 

 and no possibility is provided for the rearrangement of its contents, 

 which appears to be necessary if the half is to regulate into a whole. 

 Indeed, it is difficult to see what kind of stimulus other than in- 

 version could upset what in the uninjured blastomere is merely the 

 continuation of normal development. In one case, two frog's eggs 

 were found enclosed within one membrane, which deformed both 

 of them into a hemispherical shape. In the subsequent develop- 

 ment each embryo was deficient on the flattened side.^ 



In another anuran, Chorophilus, it has been found possible to 

 remove the injured blastomere altogether by sucking it out with a 

 fine pipette, and the uninjured blastomere then develops into a 

 whole embryo, presumably as a result of the rearrangement of its 

 contents, for after removal of its injured sister the uninjured blasto- 

 mere becomes spherical.^ Lastly, improved technique has made 



1 Schultze, 1894; G. Wetzel, 1895. ^ Morgan, 1895. 



3 Witschi, 1927. ^ McClendon, 1910. 



