886 PROGRESS OF SCIENCE IN THE CENTURY. 



primitive indifference, the absence of differentia- 

 tion, which explains the developmental potentiality 

 of the separated units of the two-celled or four- 

 celled stages. 



Somewhat similar experiments have been made by 

 other investigators on the developing ova of ascidians, 

 sea-urchins, etc. Specialisation of segmentation- 

 cells appears to occur at different times in different 

 animals, but it is illogical to infer the absence of 

 specialisation from the fact that any of the first four 

 blastomeres, let us say, can produce an entire embryo. 

 For specialised cells may retain a power of regener- 

 ation. 



(d) Pressure-Experiments. Many investigators, 

 e.g., Driesch, O. Hertwig, Born have studied the 

 behaviour of an ovum subjected to the constraint of 

 slight pressure between glass plates. Prof. Hertwig 

 shows that various compressions profoundly modify 

 the course of segmentation, the direction and suc- 

 cession of the cleavage planes, and the size of the 

 blastomeres. The nuclei may be most variably dis- 

 posed, they may lie in disorder, " like a heap of balls 

 thrown together," and yet normal embryos result. 

 This is regarded by many as a strong argument 

 against the theory that qualitatively different por- 

 tions of the nucleus are separated from one another 

 by the early cleavages. 



Here we may also refer to the interesting results 

 of rotating the eggs so that the distribution of their 

 substance is affected by "centrifugal force." This 

 may also have a profound effect on the segmenta- 

 tion ; thus O. Hertwig has shown in the case of the 

 frog's egg that the normal segmentation (total and 

 unequal) may be replaced by a process closely akin 

 to the type known as partial and discoidaL 



