82 DEVELOPMENT OF THE FROG'S EGG [Cn. VIII 



call the cleavage-axis or secondary axis. To this axis there also 

 belong an imaginary secondary equator, parallels, and meridians. 

 If the egg should be turned, after cleavage, so that neither the 

 primary nor the secondary axis is vertical, then the diameter 

 that stands at the time vertical may be spoken of as the tertiary 

 axis. 



It will be seen, from what has been said, that the imaginary 

 primary and secondary axes (with their systems) turn with the 

 egg, i.e. may be thought of as constituent parts of the egg; 

 ^yhile the tertiary axis only corresponds to any diameter of the 

 egg that is for the moment vertical. 



THE EFFECT OF GRAVITY ON THE DIRECTION OF THE 

 CLEAVAGE 



In normal eggs the first and second cleavages are vertical, 

 the third horizontal. The question arises, "Does there exist a 

 causal relation between the cleavage-planes and the egg-axis, as 

 has always been assumed without question, or do the first two 

 cleavages go through the primary axis, only because the latter 

 coincides with the force of gravity ? " This can be tested by 

 preventing the normal rotation of the egg, and Pfliiger found a 

 simple method by which this is possible. 



When the frog's egg is removed from the uterus, it is covered 

 by a thin coat of gelatinous substance which quickly absorbs 

 water and, if sufficient water is present, a space appears after 

 fertilization between the egg and its innermost membrane. 

 If an egg is taken from the uterus and placed in a dry watch- 

 glass, and only a drop of water containing sperm is added, 

 then the membrane swells somewhat, and sticks firmly to the 

 glass ; if now the right amount of water is added, the surface 

 of the egg remains in contact with the egg-membranes and the 

 egg cannot rotate as it does under normal conditions. The 

 watch-glass containing the egg may be turned in any position, 

 and the egg turns with it, so that any desired point of the egg's 

 surface may be placed uppermost. Let us imagine an egg to 

 be so turned that the black pole lies on one side. In the 

 course of three hours the first division comes in, but now the 

 plane of the first cleavage may not correspond to the primary 



