96 THE BIOLOGICAL PROBLEM OF TO-DAY 



the contents of the egg according to the weights of 

 the particles, are agencies determining the par- 

 ticular kind of development. It is no case of 

 special groups of determinants within the nucleus. 



Thus, an oval and an elongate egg produce 

 respectively an oval and an elongate blastosphere. 

 The blastosphere determines the orientation of the 

 gastrula, and so forth. In fact, the original dis- 

 tribution of mass in the material of the egg is 

 carried directly on to the following stages of develop- 

 ment (oval eggs of triton, insects, etc.). 



So, finally, in many eggs, where, in addition to a 

 polar differentiation, there is also a bilateral sym- 

 metry in the distribution of substances of different 

 specific gravities and of different physiological 

 value, the resulting blastospheres, from the reasons 

 given above, assume a bilaterally symmetrical form. 

 Although, then, in eggs with polar differentiation, 

 which have either one axis longer or are bilaterally 

 symmetrical, under normal conditions the planes of 

 the first two segmentations may correspond to the 

 principal axes of the future embryo, the cause for 

 this agreement lies in the structure of the egg, and 

 is not to be looked for, as Roux and Weismann 

 suppose, in differentiating processes of cleavage, 

 undergone by the nuclei in their first divisions. 

 It is in this way that there are to be explained the 

 investigations made by Van Beneden and Jiilin 

 upon the eggs of ascidians, by Wilson upon the egg 

 of Nereis, by Roux upon the egg of Rana esculenta, 

 and by me on the egg of Triton. 



As it fails with the process of cleavage, so Weis- 



