88 OUTLINES OF CHORDATE DEVELOPMENT 



rective effect of gravity is removed; it is not known, however, 

 what the relation of the plane of symmetry to the fertilization 

 point is under such conditions. And further, by placing the eggs 

 in positions of constraint such that the sperm entrance point 

 cannot lie in the median gravitational plane, it is found that 

 the plane of symmetiy is then that of the median gravitational 

 plane. The secondary factors may thus be independent in 

 their action, but in nature they usually tend to produce the 

 same effect. However, in nature, and this also indicates their 

 fundamental independence, there is some deviation between the 

 plane of symmetry and either the fertilization meridian or the 

 gravitational plane. The eggs in the interior of the mass are 

 subject to some constraint due to pressure, and unknown 

 factors may cause some variability in the effects of the factors 

 named in the determination of symmetry. It is possible that 

 the direction of the incident light (heat) rays plays some small 

 part in the determination of the position of this plane. So 

 that while the normal relation is that of coincidence of all 

 these, all other relations are possible (the plane symmetry of 

 course is always polar) and do occur with some frequency, and 

 all that can be said is, that on the whole the median plane of 

 the egg tends to coincide with the gravitational plane and with 

 the fertilization meridian. 



One of the reasons why the position of the plane of symmetry 

 of the egg is of the greatest importance is that the plane of 

 bilateral symmetry of the embryo and adult is directly related 

 to it. Any factor which aids in the determination of the egg 

 symmetry is at the same time influencing the symmetry of the 

 developing embryo. By observing, in a large number of speci- 

 mens, the relation between the symmetry of the egg and of the 

 embryo it is found that the tendency for the two to correspond 

 is very marked. And yet variations of any extent may and 

 do occur, showing that other factors may influence and devel- 

 opment of the embryo (Jenkinson). The symmetry of the 

 embryo and adult can be traced directly back into the gastrula 

 or blastula, and it seems, therefore, that whatever causes the 

 non-correspondence between these symmetries must operate 



