72 ORIGIN OF POLARITY, SYMMETRY, AND ASYMMETRY 



of the Gastropod egg is of the spiral type, and it has been shown 

 that the direction of the initial spiral cleavage division is opposite 

 in dextral and sinistral races of snails.^ It is therefore probable that 

 the orientation of the spindles of the first spiral cleavage division 

 is responsible for the determination of the type of asymmetry which 



<^^ 



Fig. 30 



Above, a sinistral (left) and a dextral (right) Gastropod shell. Below, corre- 

 sponding asymmetry of cleavage. The obliquity of the spindles in the 2-cell 

 stage (centre) and the portion of the cross-furrows in the 4-cell stage (bottom) 

 are reversed in eggs with the left-handed (laeotropic) and the right-handed 

 (dexiotropic) type of cleavage. (After Morgan, Experimefttal Embryology (Figs. 80, 

 79, 78 c and c\ pp. 256-7), Columbia University Press, 1927, modified.) 



the adult will exhibit,- and that the orientation of the spindles 

 is, in turn, controlled by the Mendelian factors present in the 

 oocyte (figs. 29, 30). 



Reversed spiral cleavage has been observed exceptionally in the 

 development of the leech Clepsme,'^ but as the adult is apparently 

 perfectly symmetrical, no subsequent effects of the reversed cleav- 

 age can be detected. Occasionally, the leech egg gives rise to a 



1 Crampton, 1894. - Conklin, 1897. ^ Miiller, 1932. 



