110 Reversal of Cleavage in a Sinistral Gasferojwd. 



EXPLANATION OF PLATE Y. 



Fig. 1. — 4-cell of Limnaea, from the animal pole. 



Figs. 2, 3, 4, 5. — 4-, 8-, 12, and 16-cell stages of Physa, all from animal 

 pole. I-I, first cleavage plane ; II-II, second cleavage plane ; A, B, C, D, 

 macromeres : a^, 6', c', d}, first group of micromeres ; cfi, h^, c^, (P, second 

 group of micromeres ; o^-^ o^-^, i^-i, li^-^, etc., descendants of the first group of 

 micromeres. The arrows show the direction of the spiral division. 



