92 



CLEAVAGE AND DIFFERENTIATION 



The result of this experiment was at first interpreted to mean that 

 the two blastomeres were already differentiated at the first cleavage 

 and were determined to give rise, each of them, to one-half of the 

 future embryo. But this conclusion was later shown to be errone- 

 ous in a number of ways. In the first place, it was noticed that the 

 half-embryo which developed might be a lateral half, or a dorsal 

 half, or an oblique half, according as to whether the plane of the 



A, Lateral, and B, anterior, partial embryos of the frog produced from eggs in 

 which one of the first two blastomeres have been killed but allowed to remain in 

 place. (After Roux, from Morgan, Experimental Embryology, Columbia Uni- 

 versity Press, 1927.) 



first cleavage coincided with, or was perpendicular, or oblique, to 

 the plane of bilateral symmetry. The alleged determination of the 

 blastomeres at the 2-cell stage was therefore not constant.^ Then 

 it was found that if a normal embryo at the 2-cell stage is inverted, 

 each of the two blastomeres will then develop into as much as it 

 can of a complete embryo. The limitations on completeness are due 



uninjured half, or by a combination of these methods. Morgan (1895) was unable 

 to confirm these findings, and the position is still obscure. Discrepancies be- 

 tween various results seem to be due to the relative degrees of injury inflicted by 

 the hot needle. Where the coagulation of the protoplasm is extensive and cleavage 

 of the injured blastomere cannot proceed, it is unlikely that the half-embryo ever 

 becomes complete, although it may appear to be more complete than it really is, 

 as a result of the spreading of the epidermis from the uninjured half and con- 

 sequent concealment of the underlying defects. If, on the other hand, the cleavage 

 of the injured blastomere is only delayed but it nevertheless reaches the blastula 

 stage by the time that the uninjured half is ready to gastrulate, the rapid restora- 

 tion of the missing half would be possible. At all events, the theoretical arguments 

 originally based on the alleged phenomenon of post-generation have long ceased 

 to be important. 



^ Hertwig, 1893; Brachet, 1903, 1905. 



