THE EARLY DEVELOPMENT OF THE FROG 87 



and the egg is incapable of perfect regulation under abnormal 

 conditions imposed upon it, so that artificial lesions or other 

 disturbances result in abnormalities of its structure which lead 

 to abnormalities in cleavage or embryonic development. 



To summarize, the bilateral symmetry of the egg is deter- 

 mined primarily by the polarity of the egg which has gradually 

 developed during its formation and maturation. How exten- 

 sive this essential bipolar structure is, we do not know, but it 

 is expressed visibly, and probably only in part, by the polar 

 arrangement of at least three different substances having dif- 

 ferent specific densities protoplasm, pigment, deutoplasm. 

 Probably the arrangement of these is itself determined by 

 some fundamental structure of the egg, but this we cannot 

 observe directly. The bilateral symmetry is here only poten- 

 tial. It becomes actual only after ensemination when these 

 substances are rearranged, first under the influence of the en- 

 tering spermatozoon, which brings about the non-correspond- 

 ence of the egg axis and gravitational axis, and then through 

 the influence of gravity according to the plane fixed by these 

 two crossing axes. In other words the bilateral symmetry is 

 first determined by the egg structure as expressed through its 

 polar differentiation and then through that as the result of the 

 action of the entering spermatozoon and gravity, which latter 

 is able to act finally only after the entrance of the sperm. 



The penetration path of the sperm is usually in the direction 

 of a radius of the egg from the entrance point, so that this 

 portion of the sperm path tends to lie in the plane of symmetry 

 to the same extent as, or even to a greater extent than the 

 entrance point itself, and it may be that we should express 

 the relation more truthfully by saying that the plane of sym- 

 metry tends to be directed first by the location of the pene- 

 tration path rather than the fertilization point, since the influ- 

 ence of the sperm is exerted as it passes all the way along this 

 portion of its course. 



By placing the eggs under artificial conditions it has been 

 found that the action of both these secondary factors is not 

 essential, for normal development proceeds even when the di- 



