No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 1 5 



blastomeres being displaced or rotated with respect to the egg- 

 axis, either to the right, following the hands of a watch (right- 

 handed spiral), or in the reverse direction (left-handed spiral), as 

 the case may be." " The term ' spiral ' refers to the fact that 

 the curved radii, if prolonged, would form a spiral about the 

 egg-axis." In the ontogeny there is no twisting of the radii, 

 but merely an inclination of the axis of the dividing cell from 

 the vertical. It seems to me, therefore, that this form of 

 cleavage would be more correctly termed oblique. 



In the radial type cleavages are either vertical or horizontal 

 with respect to the egg-axis ; the cleavage spindles are hence, 

 respectively, horizontal or vertical. In the oblique type 

 the second cleavage spindle is not horizontal, but oblique. 

 From the point of view of an observer in the axis of 

 the ^%g, the spindle is inclined from right below to left 

 above, which can be expressed by the single word leiotropic. 

 In the third cleavage the spindle is dexiotropic. Regarding 

 the ovum from the animal pole, the upper cell lies to the left 

 of the lower, i.e., in a leiotropic position, in the first instance, 

 and to the right, i.e., in a dexiotropic position, in the second 

 instance. The second cleavage of the macromeres is leiotropic, 

 and the third dexiotropic again. In the following pages the 

 cleavages will be described as leiotropic or dexiotropic accord- 

 ing as the inclination of the cleavage spindle from below above 

 is to the left or right of the vertical axis of the ovum, and 

 not according to the direction of the actual planes of division. 



Crampton (No. 41^) has discovered that in Physa, a sinistral 

 pulmonate, the directions of the cleavages are reversed. Thus, 

 the spindle in passing from the four to the eight-cell stage 

 is leiotropic, not dexiotropic as in the other cases cited. 

 Inasmuch as the obliquity changes from right to left, or vice 

 versa, with each successive cleavage, as in the other cases, the 

 mesoblast is formed from the nght, not the left, posterior 

 macromere. The direction of the division of the left posterior 

 macromere at the fourth cleavage would throw its product on 

 the left side of the embryo, while the homonymous product 

 of the right posterior macromere is thrown in the middle line 

 behind. It would seem in this case that the position of the 



