308 SCIENCE PROGRESS. 



of the heap of balls is a very loose one. To understand 

 the meaning of the summary one must turn to pp. 678-685 

 of the same memoir, which consist of a section entitled 

 " Erklarung des abnormen Furchungsverlaufes ". There 

 we learn, as we had previously learnt from Driesch, that 

 the divisional planes of segmenting ova are determined by 

 the direction of the nuclear spindles and that the orientation 

 of the first nuclear spindle is determined by the character of 

 the body of the ovum and its contents. The ova of Echinus 

 are homogeneous throughout, and orientation of the first 

 nuclear spindle is a chance affair. But the ovum of the 

 Frog is not homogeneous ; it consists of a smaller cap of 

 protoplasm resting on a large body of yolk, and the nucleus 

 lying in the cap of protoplasm, the direction of the first 

 nuclear spindle is determined by its relations to the more 

 active yolk on the one hand, and the denser food yolk on 

 the other. The relations of the food yolk and protoplasm 

 are changed by the pressure applied during the experiments 

 and the changes are different according as the pressure is 

 applied vertically or horizontally. Hence the direction of 

 the first and the succeeding nuclear spindles is changed 

 in different senses, according to the pressure employed. As 

 the divisional planes are always at right angles to the 

 nuclear spindles, the positions of the two first and the suc- 

 ceeding blastomeres differ according as the pressure applied 

 is vertical, horizontal, oblique, or circumferential. One may 

 in fact cause the blastomeres and their contained nuclei to 

 take up what position one will by varying the direction of 

 the pressure. In this sense, and in this sense only, can one 

 speak of rolling the nuclei about like balls. Not a word about 

 a protoplasmic mass through which the nuclei are caused to 

 roll. On the contrary, a great deal about planes of division 

 and splitting up of the egg into corpuscles round the nuclei. 

 It only requires a glance at Hertwig's figures and diagrams 

 to show that the blastomeres are as distinct during abnor- 

 mal division as during normal division, and that there is not 

 at any time any question of a "protoplasmic mass," a cir- 

 cumstance which has been well understood by everybody 

 who has taken the trouble to read his memoir carefully. 



