230 REGENERATION 



prospective value of a blastomere is a function of its position. 1 

 Driesch extended these experiments further in 1893. His aim was 

 to separate different groups of cells at the sixteen-cell stage in order 

 to see whether the cells around the micromere pole (or "animal-pole ") 

 if separated from those of the opposite (or "vegetative pole") could 

 produce a whole embryo, etc. Eggs whose membranes had been re- 

 moved by shaking immediately after fertilization were allowed to 

 develop normally to the sixteen-cell stage and were then shaken into 

 pieces. Amongst the groups of cells that were present those that con- 

 tained the micromeres were picked out. It was found that they give 

 rise to whole embryos. In order to obtain cells that belong to the 

 vegetative hemisphere, the blastomeres were shaken apart at the 

 eight-cell stage, and those groups of cells that in later divisions did 

 not produce micromeres were isolated. From these also whole em- 

 bryos develop. The results show that the cells of both hemispheres 

 are able to produce whole embryos, and that at the sixteen-cell stage 

 the different parts of the egg are still capable of producing all parts 

 of the embryo. It is important to observe that the results of the 

 experiment do not show that if the normal development goes on 

 undisturbed any part of the egg may become any part of the em- 

 bryo, for it is highly probable that a definite region of the egg may 

 always produce a definite part of the embryo. The results do show, 

 however, that, even if this is true, any cell has the power of produc- 

 ing any or all parts of the embryo if the normal conditions are 

 changed. 



In connection with these experiments Driesch discussed the fac- 

 tors that determine the axial relations of the embryo. If all the cells 

 have the power of producing all parts, what determines in the normal 

 development, and also in the development of a part of the whole, the 

 axial relations of the embryo ? Driesch assumed that the egg has a 

 polar structure, and that the same polarity is found in all parts of the 

 protoplasm. Around this primary axis all the parts are alike or 

 isotropous. 2 The origin of the mesenchyme and the position of the 

 archenteron, that develop at one pole, are determined by the polarity 

 of the protoplasm. The plane of bilateral symmetry may appear in 

 any one of all the possible radial planes around the primary axis. 

 The selection of a particular one is due to some accidental difference 

 in the structure of the protoplasm, or to some external factor. In 

 later papers Driesch modified this view, and assumed that along with 

 the primary polarity a bilateral structure also exists in the proto- 

 plasm. 



1 Hertwig had a year before expressed a similar view in regard to the equivalency of the 

 blastomeres. 



2 A view advanced by Pfliiger. 



