84 OUTLINES OF CHORDATE DEVELOPMENT 



bilateral symmetry always passes through the chief egg axis. 

 The secondary factors then determine through what meridian 

 the plane will pass. One of the secondary factors is internal 

 and one external; the former is the point of entrance of the 

 spermatozoon together with the direction of its penetration 

 path, the latter is the direction of the action of gravity with 

 respect to the egg axis. These secondary factors alone cannot 

 direct the position of this plane, but each acts only in connec- 

 tion with the primary factor which is, in reality, the essential 

 structure or organization of the egg as expressed by its polarity 

 and rotatory symmetry. 



We should recall that the sperm enters the upper pole in any 

 meridian (fertilization meridian) and that its penetration path 

 is first approximately radial, while the latter part of its path, 

 copulation path along which it passes after the sperm head 

 has dissolved and become vesicular, may be at an angle with 

 the penetration path. 



Immediately upon the entrance of the spermatozoon the 

 substance of the egg becomes more labile, and a sharper dif- 

 ferentiation and more pronounced segregation of the various 

 egg substances result. It is supposed that the influence of the 

 sperm is first exerted in the cytoplasm in its own immediate 

 neighborhood, and that the effects of its presence then spread 

 gradually to the more remote parts of the egg; and further, 

 that the influence of the sperm extends in a symmetrical wave 

 like those from a vibrating body. The result of this would 

 be that the rearrangement of the substances of the egg would 

 be symmetrical with reference to the point of origin of the 

 disturbance, namely, the sperm entrance point. Therefore the 

 plane of symmetry of the egg would be that plane containing 

 the three points: animal pole, vegetal pole, sperm entrance 

 point (Fig. 27, B). This would be at the same time the plane 

 containing the penetration path, and it would be marked 

 superficially as the fertilization meridian. 



Whether or not this is a true description of the effects of the 

 sperm, the facts are that following impregnation there is a 

 streaming of the protoplasm upward and of the deutoplasm 



