356 THE CELL 



nucleus and the specifically lighter portion of the egg have been 

 forced to change their position, the first division plane takes anew 

 a vertical direction. This plane cuts the horizontal egg-axis at 

 various angles. For instance, Pfliiger often saw that it separated 

 the egg into a black and a white hemisphere. Under such cir- 

 cumstances, therefore, the hemispheres evidently do not contain 

 the same particles of material, as when they are under normal 

 conditions. Nevertheless, a normal embryo is developed out of 

 the egg. Even after the formation of the notochord and spinal 

 cord, one half of the body can be seen to be darker than the 

 other. Thus, according to the position of the original cleavage 

 plane, the individual organs must be composed of different parts 

 of the egg contents. The experiments made by Richard Hertwig 

 and myself (VI. 38), by Boveri (IX. 4), by Driesch (IX. 7), and 

 by Chabry (IX. 5), all furnish additional proof of the isotropy of 

 the egg. 



Richard Hertwig and I found, that the ova of Echinoderms can 

 be divided by violent shaking into small portions ; these become 

 spherical in form, and may be fertilised by spermatozoa. Boveri 

 indeed has succeeded in raising a few dwarf larval forms from 

 such small fertilised portions. Driesch, by shaking normally 

 developed and dividing Echinoderm ova, was 'able to separate 

 from one another the two first cleavage segments ; these he then 

 isolated, and was thus able to establish the fact that a normally 

 shaped though somewhat small blastula, followed by a gastrula, 

 and even in some cases by a pluteus, developed from each half. 



Chabry has obtained a corresponding result. He destroyed, by 

 pricking it, one of the two, or, when it had divided into four, one 

 of the four cells of the ovum of an Ascidian. In many cases he 

 succeeded in raising from such mutilated ova, absolutely normal 

 larvae, which only occasionally, were without subordinate organs, 

 such as otoliths or attachment papillse. From all these experi- 

 ments the fundamental proposition is proved, that the cell-nucleus, 

 which may be enclosed in any part of the yolk, is able to produce 

 a complete organism. This isotropy of the egg negatives the 

 hypothesis that there is a germinal region from which organs are 

 developed. Moreover, at the same time, it supplies an additional 

 proof that the idioplasm is not to be found in the protoplasm, but 

 in the nucleus; and further, it allows us to draw some conclu- 

 sions as to the construction of protoplasm and nuclear substance. 



Protoplasm must consist of loosely-connected particles of mi- 



