ORIGIN OF POLARITY, SYMMETRY, AND ASYMMETRY 77 



plane of bilateral symmetry is already determined at fertilisation, 

 rotation of the egg so that the vegetative pole becomes uppermost, 

 while the dorsal meridian remains unchanged, would cause the 

 original left side of the egg to become the right side of the embryo. 

 But as a matter of fact, the embryos arising from such reversed eggs 

 do not show situs inversus.^ 



The asymmetry factor must therefore be regarded as a factor 

 which results in a greater activity of the tissues on one side (the 

 left) of the body as compared with the other : such greater activity 

 developing progressively. It is interesting to note that a similar 

 progressive accentuation of a differential or gradient is to be ob- 

 served in the dorso-ventral axis of the amphibian egg, between the 

 time when the grey crescent is first formed and the establishment 

 of full organiser capacity in the dorsal lip (see p. 68). Further 

 research should be directed to discovering whether such accentua- 

 tion or steepening of activity-gradients is a regular feature in their 

 development. 



The result of the action of this asymmetry factor is seen in the 

 more rapid growth and differentiation of the left side, as regards 

 certain organs. Experimental proof of this is provided by the meso- 

 dermal rudiments from which the muscular wall of the heart is 

 formed. These rudiments are at first situated on each side of the 

 body, and later on move towards the middle line. But if the rudi- 

 ments are removed from the embryo while they are still lateral in 

 position, and are made to develop in isolation (explantation in ecto- 

 dermal jackets, in suitable culture media), the remarkable thing is 

 that the rudiments from the left side show pulsations while those 

 from the right do not. Further, the histological differentiation of 

 which the rudiments from the right side are capable is inferior in 

 degree to that of the rudiments from the left.^ The difference be- 

 tween the left and right rudiments of the heart is a physiological 

 one, and appears to be quantitative rather than qualitative, and in 

 every case the left side is prepotent (fig. 33). 



If the asymmetry factor is, as suggested, concerned with the 



relatively greater activity of one side, it should be susceptible of 



experimental control. One method of affecting its action is the 



simple mechanical one of removing some of the material from the 



^ Hammerling, 1927. " Goerttler, 1928. 



