58 THE BIOLOGICAL PROBLEM OF TO-DA Y 



and of Roux has been shown by varied ex- 

 periments of my own. The eggs of frogs on the 

 point of cleaving were flattened to a disc between 

 vertically or horizontally placed glass-plates. In 

 the first case they were flattened in the dorso- 

 ventral direction, i.e., the axis passing through the 

 animal and vegetative pole was shortened ; in the 

 second case an axis at right angles to this was 

 shortened. In both cases the course of cleavage, 

 and the resulting distribution of the nuclei in the 

 yolk, was artificially modified. 



The diagrams A, B, C (Fig. 3) will make the 

 results plain to the reader. A, represents the dis- 

 tribution of the nuclei after normal cleavage ; 

 B, the same, when the egg was pressed between 

 horizontally- arranged parallel glass-plates ; C, the 

 same, where the flattening was produced by 

 vertically-placed parallel glass-plates. 1 



The diagrams show the positions of the seg- 

 mentation spheres and of the contained nuclei as 

 seen from the animal pole. In stages where two 

 layers of cells as a result of division lay one above 

 the other, the cells of the lower layer are distin- 

 guished in the figure by shading. In the three 

 diagrams the nuclei are numbered so that the 

 reader may know how far they are removed from 

 the nuclei of the first two segmentation spheres. 

 The numbers are further exhibited in the following 

 two genealogical trees : 



1 The terms vertical and horizontal refer to the vertical axis of 

 the egg, which passes through the animal and vegetative poles. - 

 Translator s note. 



