22 



BIOLOGICAL LECTURES. 



gastrular axis and the cephalic and oral poles of the larva. In 

 many cases the antero-posterior axis is marked out by the 

 inequality of the first cleavage, and this is preceded by the 

 eccentricity of the nuclear spindle, which in turn must be 

 the result of the structure of the unsegmented Q^g. The 

 direction of the first cleavage in Crepidula and probably in the 

 other cases mentioned is always dexiotropic, that is, of such 

 a character that the nuclei and protoplasmic areas of the two 

 resulting cells rotate in a clockwise direction at the close of 

 the cleavage (Fig. i). This character must also be predeter- 

 mined in the unsegmented ^%%. It is the first of a long series 



Fig. I. — Crepidula, 2-cell stage ; showing dexiotropic rotation of nuclei asters and cytoplasm 



at the close of the first cleavage. 



of "spiral cleavages" (Figs. 2, 3, 4) which are oblique alter- 

 nately to the right and to the left, each of which, except the 

 first, finds the cause of its direction in that of the preceding 

 cleavage. The direction of these cleavages stands in the most 

 intimate relation to the origin of the mesoblastic pole cells, the 

 appearance of bilateral symmetry, and the direction of the 

 asymmetry of the adult. In all cases in which the first cleav- 

 age is dexiotropic the pole cells of the mesoblast arise from 

 the left posterior macromere by laeotropic division (Fig. 4); 

 where the first cleavage is laeotropic (as in some sinistral gas- 

 teropods) they arise from the right posterior macromere by 

 dexiotropic division. In Crepidula bilateral symmetry appears 

 in different directions in the ectoblast, mesoblast, and entoblast, 

 and by a subsequent laeotropic rotation, which is dependent 



