No. I.] THE EMBRYOLOGY OF CREPIDULA. 183 



(b) The Bilateral Type. 



In purely bilateral cleavage, such as is found among the 

 ascidians and cephalopods, the very first division is bilaterally 

 symmetrical. In other cases bilaterality may not be clearly 

 marked at the beginning of the development, but still may 

 appear at a very early stage. 



Wilson ('93) has found in Amphioxus that the cleavage of 

 normal eggs may be bilateral or spiral or radial in the earliest 

 stages, although there is shown "a distinct tendency toward 

 bilaterality in almost all forms of the cleavage." 



In annelids, gasteropods, and lamellibranchs, on the other 

 hand, the cleavage is typically spiral until about the time of 

 the formation of the mesoblast (4d). In Nereis, according to 

 Wilson, this spiral cleavage suddenly gives place to the bilat- 

 eral. "The most striking feature in the cleavage," says 

 Wilson, " and the one on which the entire discussion may be 

 made to turn, is the sudden appearance of bilateral symmetry 

 in the cleavage; . . . the bilaterality does not appear at the begin- 

 ning of development. It appears only at a comparatively late 

 stage, and by a change so abrupt and striking as to possess an 

 absolutely dramatic interest." "The bilateral asymmetry of 

 the early stages depends mainly upon the fact that the sub- 

 stance of the somatoblasts (i.e., the mesoblast and the material 

 of the ventral plate) is stored in the left posterior macromere. 

 Bilateral symmetry is established upon the reduction of this 

 macromere (D) to the size of its fellow (C) by the separation 

 of the somatoblasts and their transportation to the median line. 

 Immediately upon this event follows the appearance of bilateral 

 cleavages throughout the embryo, except in the cells which 

 give rise to the prototroch, a purely larval organ." 



Among the mollusks conditions are very different, bilateral 

 cleavages appearing very slowly, and creeping, as it were, from 

 cell to cell and from quadrant to quadrant. In Crepidula 

 they first appear at the 34-cell stage and by a slight shifting 

 in position of a single cell, 3d2, immediately after its forma- 

 tion ; so slight is this movement that it is doubtful whether it 

 ought to be considered as indicative of bilateral cleavage. The 



