No. 2.] THE DEVELOPMENT OF THE NEWT. 325 



room for expansion ; on all other sides the o^gg is hemmed 

 in and is sometimes slightly compressed, as in cases where 

 the capsule is so narrow as not to permit the Qgg fully to 

 regain its spherical form. It is a curious coincidence, if 

 nothing more, that the expansion of the Qgg incident upon 

 the first cleavage should occur in just that direction m which 

 it is most free to expand, that in other words the spindle takes 

 up that position in which it can act most freely. 



I am inclined to accept provisionally some such mechanical 

 interpretation of the determinateness of direction of the first 

 cleavage plane in the newt's ^gg. I do this the more readily 

 since the effect of pressure in influencing the direction of 

 cleavage planes is well known through the researches of 

 Pfliiger, Roux, Driesch, and others. Roux's experiment of 

 elongating frogs' eggs by compression in a narrow glass tube 

 showed that such elongated eggs divided at right angles to their 

 long axis, a fact that bears directly upon the case in hand. 



By attributing the direction of the first cleavage plane to 

 Sachs' law I do not in the least wish to be considered as imply- 

 ing that in the eggs of some animals other causes may not often 

 take the place of pressure as a determining factor and may 

 indeed override it. I wish simply to state my belief that 

 mechanical compression operates as the determinant of the 

 direction of the first cleavage plane in the newt. 



If we admit, as I think we are bound to do, that the earliest 

 cleavage stages of these eggs have little or no morphological 

 significance then I see no escape from the conclusion that 

 they must depend on environmental or mechanical causes of 

 one sort or another. Precocious differentiation may invest the 

 early cleavage-stages of some eggs with a factitious significance. 

 In others the early spindles may change the direction of their 

 axes under the influence of various slight mechanical causes. 



V. Formation of the Blastopore. 



Technique. — The difficulty of extricating the living Qgg unin- 

 jured from the membrane has been so great that I have as a 

 rule preferred to kill the Qgg while it was still in the envelopes. 



