90 OUTLINES OF CHORDATE DEVELOPMENT 



considerably by the direction of the copulation path of the 

 sperm nucleus, i.e., the direction of the plane passing through 

 the contact surface of the copulating egg and sperm nuclei, 

 and since this is subject to much variation with respect to the 

 median plane, and also since the postion of the spindle is easily 

 modified by external factors, we find that the relation between 

 the first cleavage and the median plane of the egg is not at 

 all regular, and deviations from the two relations mentioned 

 are very frequent and all relations occur. There is no direct 

 relation between the plane of the first furrow and the fertili- 

 zation meridian; whatever relation there is results from their 

 common relation to the plane of egg symmetry. 



We may summarize briefly the relations of the plane of sym- 

 metry of the egg, the plane of symmetry of the embryo, and 

 the plane of the first cleavage furrow, in normally developing 

 eggs. 



The position of the plane of bilateral symmetry of the egg is 

 determined primarily by the polarity and rotatory symmetry 

 of the unfertilized egg, in conjunction with the point of entrance 

 of the sperm or the direction of the penetration path, and the 

 direction of the action of gravity, in such a way that the 

 median plane tends to lie in the gravitation plane, which also 

 tends to coincide with the fertilization meridian. This determi- 

 nation, however, is not complete and variations may and do 

 occur. 



The median plane of the bilaterally symmetrical embryo 

 tends to a marked degree to coincide with the plane of symmetry 

 of the fertilized egg, but all other relations in the same axis 

 are possible and actually occur. 



The plane of the first cleavage furrow tends to lie either in 

 or at right angles to the plane of symmetry of the egg, primarily 

 on account of the tendency of the first cleavage spindle to 

 assume some symmetrical position with reference to the egg 

 structure. 



There is, therefore, a tendency for the gravitational plane, the 

 point of entrance of the spermatozoon, the penetration path of 

 the spermatozoon, the median plane of the egg, the median 



