ORIGIN OF POLARITY, SYMMETRY, AND ASYMMETRY 8l 



these, also, it is the left side of the body which is prepotent as com- 

 pared with the right, and this prepotency manifests itself in the fact 

 that the hydrocoel, the water-pore, and the rudiments of the adult 

 animal are formed on the left side of the body of the larva. In 

 Asterina, the gastrula can be divided into left and right halves by 

 section in the plane of bilateral symmetry. The left halves develop 

 into larvae with normal asymmetry : they have a hydrocoel on the 

 left. The right halves can do one of three things ; they may have a 

 hydrocoel on the left side only ; they may have a hydrocoel on the 

 right side only ; or they may have a hydrocoel both on the left and 

 on the right side.^ This last condition is sometimes found in other- 

 wise normal echinopluteus larvae,'^ and can be experimentally 

 produced by subjecting the larvae to hypertonic sea-water,^ while 

 it is the rule in ophioplutei. 



The occurrence of halves produced from the right side of the 

 original larva, which develop a hydrocoel both on the left and on 

 the right, is of great interest, for it provides a situation which could 

 scarcely be realised in the amphibian embryo. There, the gut and 

 heart must be twisted either one way or the other, but cannot be 

 twisted in both ways at once in the same embryo.^ 



Both this result on Echinoderm larvae, however, and that ob- 

 tained by dividing newt blastulae into right and left halves (p. 76), 

 can be plausibly explained along similar lines. It has already been 

 found necessary to postulate a main activity-gradient, concerned 

 with asymmetry, and extending transversely across the body from 

 left to right. This is presumably superposed on minor activity- 

 gradients extending inwards from the surface towards the centre of 

 the embryo. In any case, when the developing tgg is cut in half, the 

 inner surfaces of each half are damaged or interfered with, and their 

 activity reduced. In the left-hand halves, the effect of this will 

 merely be to steepen the existing asymmetry-gradient ; all resulting 

 organisms will therefore be of normal asymmetry. In the right-hand 

 halves, however, the effect will be in the contrary direction to that 



1 Horstadius, 1928. 2 MacBride, tgii. 3 MacBride, 1918. 



* In the Gastropod Limncea (see p. 411) occasional specimens have been bred 

 in which the dextral and sinistral forces are so delicately balanced that the result 

 is an animal with a flat shell coiled in one plane, like that of Planurbis. Most of 

 these specimens are abnormal in their anatomy and die early (Boycott, Diver, 

 Garstang and Turner, 1930). 



