1894.] Embryology. 273 
ing the slide vertical and allowing the eggs to take up their normal 
position before the second slide is pressed upon them, the cleavage is 
abnormal. The second plane is not a vertical one but is horizontal so 
that two black-pole cells and two light-pole cells are formed. The two 
former cells are very small and divide up by somewhat vertical planes 
parallel to the first. Thus the second, normal, plane remains long 
absent. When the plates are inclined to 45° a still different modifica- 
tion of cleavage results. 
The eggs that are drawn into narrow tubes are distorted into cylin- 
drical or barrel-shaped masses that cleave abnormally. When the tube 
rests horizontally the first plane is vertical or normal but always at 
right angles to the axis of the tube, the second is normal, that is, at 
right angles to the first, but the third is also vertical and not horizon- 
tal: the fourth is horizontal. 
When the tube is placed vertically the black part of the egg is up- 
permost and the cleavage is again altered by the pressure of the tube. 
The first plane is oblique and variable, but divides off a smaller upper 
cell from a larger lower cell. 
All these abnormal modes of cleavage may, the author maintains, 
be explained upon his principle that the cleavage plane is at right 
angles to the axis of the nuclear spindle and that the position of the 
spindle-axis is dependant upon the shape and character of the proto- 
plasm about it; the poles of the spindle lie in the directions of the 
greatest masses of protoplasm. Pressure acts by changing the shape 
of the protoplasmic mass and thus inducing a new direction for the 
nuclear spindles. That in the frog different forms of cleavage result 
when the egg is pressed from the side or from above downward is to be 
explained by the quality of the protoplasmic masses, the nature of the 
Protoplasm, admixture of yolk, etc. being a factor as well as its mass 
in regulating the direction of the nuclear spindle. This explanation 
is thus more fundamental than the principles of surface tension and 
rectangular intersections of cleavage planes, which follow in part from 
this action of mass upon nuclear arrangement. 
If the eggs remain under pressure between the plates or in the tubes 
they continue to develop, form gastrulas and, in some cases, larve. 
This furnishes a good means of confirming the contention of Pfluger 
and of Roux that the medullary folds really are formed upon that side 
of the egg which is at first the light colored lower side though they © 
normally appear upon the upper side and would hence be naturally 
regarded as formed from the black or animal-pole side. 
