ORIGIN OF POLARITY, SYMMETRY, AND ASYMMETRY 71 



It is clear that a fandamental difference must exist between the 

 eggs of bilaterally symmetrical and those of radially symmetrical 

 forms. In the former case, however, what is given by genetic con- 

 stitution cannot be bilateral symmetry per se, but the capacitv of 

 developing such symmetry in relation to various external agencies. 

 Harrison (1921 A, 1925 a) has suggested that the ultimate capacity 

 for developing symmetry-relations is linked up with the intimate 

 properties of the protoplasm and the "space-lattice " formed by its 

 constituent parts. 



The asymmetry of the large chelae found in many Crustacea 

 either in one or both sexes, and also that of the opercula in certain 



a 



Fig. 29 



Cleavage asymmetry in Molluscs. The position of the large mesoderm cell (^d) is 

 reversed in laeotropic and dexiotropic cleavage. (From Morgan, Experimental 

 Embryology, Columbia University Press, igzj.) 



tubicolous Annelid worms, involves special problems of relative 

 growth-rate, which are discussed by Przibram (193 1 a). 



The most marked asymmetry known is that of Gastropod Mol- 

 lusca, most of which manifest a marked torsion of the internal 

 anatomy together with unequal development of many paired organs. 

 In addition, a large number of forms have their shells twisted into 

 a spiral, which is usually dextral. Here it has been shown that the 

 dextral or sinistral type of structure is under the control of Men- 

 delian factors, whose action, however, is delayed for a generation^ 

 (see Chap. xii). The asymmetry of the a^dult is determined not by its 

 own genetic constitution, but by that of the oocyte from which it 

 arose, before it underwent the reduction divisions. The cleavage 



^ Boycott, Diver, Garstang, and Turner, 1930. 



