ORIGIN OF POLARITY, SYMMETRY, AND ASYMMETRY 79 



Now, since the embryos that develop from eggs in which the 

 plane of bilateral symmetry has been artificially selected (by con- 

 trolling the point of sperm-entry; see p. 36) show the normal 

 asymmetry, it follows that the determination of the left as the ulti- 

 mately prepotent side must be made at the same time as that of the 

 plane of bilateral symmetry. And if this determination were due 

 to an external factor, it would be impossible to understand why it 

 invariably acts so as to produce its effect on a meridian 90° right 

 of the meridian of sperm-entry, and convey a greater power of 

 activity to this eventual left side of the embryo. The conclusion is 

 therefore enforced that the determination of the left-right axis is 

 in some way connected with that of the plane of bilateral symmetry, 

 and is the result of some factor acting within the egg. 



There is as yet no indication of how this factor acts, but it may 

 be pointed out that the determination of a third axis (in the case of 

 the egg, the left-right axis) as a consequence of the determination 

 of the other two axes of space (in the egg, the primary egg-axis, 

 i.e. antero-posterior, and the dorso-ventral), is a phenomenon not 

 without an analogy in the inanimate world. It is well known that 

 if a conductor carries an electric current through an independent 

 magnetic field of force which is orientated at right angles to the 

 conductor, then the conductor will be subjected to a force acting 

 at right angles both to the magnetic field and to the conductor. If 

 it be imagined that the magnetic field is vertical with the North 

 Pole uppermost, and a horizontal conductor carries an electric 

 current away from an observer, the force acting on the conductor 

 will tend to displace it to the observer's left. It is not pretended 

 that the egg-axis is the site of a magnetic field, nor that the dorso- 

 ventral axis is a simple conductor. But the physical analogy de- 

 scribed above does show how it is possible to obtain a determina- 

 tion of a third axis, and a polarisation in it, as a consequence of the 

 determination and polarisation of axes in the other two planes of 

 space. 



In the larva of Amphioxus, asymmetry is very marked. In this 

 form^ double monsters can be artificially produced by disarranging 

 the 4-cell stage. In such cases, both components always show the 

 normal asymmetry : symmetry is never reversed. The diflPerence 



^ Conklin, 1933. 



