102 DEVELOPMENT OF THE FROG'S EGG [Cn. X 



as to the succession of divisions. The value of this statement 

 will be discussed later. 



These experiments show clearly that by changing the form 

 of the egg, we change at the same time its method of cleavage. 

 Again, reasoning from these "induced" forms back to normal 

 forms of cleavage, we see also something of the forces at work 

 there. Pfliiger did not fail to see the importance of these 

 experiments. He believed, as we have seen, that the direction 

 of the cleavage-planes results from the direction of the pressure, 

 because when the nuclear spindle of the egg or of a blastomere 

 forms, the spindle elongates in the direction of least resistance, 

 that is, at right angles to the direction of the pressure. 



The spindle in the egg axially compressed cannot lie at 

 right angles to the plates because of the resistance of the yolk 

 below, but it must elongate in a plane parallel to the plates. 

 Since the cleavage of the protoplasm takes place at right 

 angles to the long axis of the nuclear spindle, the division- 

 planes must appear at right angles to the plates. Born has 

 pointed out that this interpretation of Pfliiger cannot be the 

 true one, because the egg is not a solid elastic ball, but a fluid 

 globe with an elastic coat. The pressure, therefore, will be 

 quickly equalized in all directions, and cannot act during the 

 time of cleavage in any given direction. 



Sachs's law for the direction of new cleavage-planes seems to 

 apply to the compressed eggs. According to Sachs, the form 

 of the whole mass determines the position of the cleavage- 

 planes. Hertwig refers the processes of cleavage directly to 

 the changes that take place in the nucleus. He thinks that 

 the nucleus tends to assume the centre of its sphere of activity, 

 which is the centre of the protoplasmic mass. This is not neces- 

 sarily the centre of an egg in which the yolk is unequally 

 distributed. Hertwig thinks that the nuclear spindle will then 

 elongate in the direction of the greatest protoplasmic mass. If 

 we apply Hertwig's hypothesis to the segmenting frog's egg, 

 we see that it appears to explain in part the various phe- 

 nomena. In the egg compressed in the direction of its pri- 

 mary axis and with the primary axis vertical (category 1), 

 the greatest protoplasmic mass will be, for the first spindle, in 

 a horizontal plane ; similarly for the second spindle. Hence 



