526 PROCEEDINGS OF THE ACADEMY OF [DeC, 



Planocera it is D, B {A, C), the latter two cells dividing at about the 

 same time. It is evident that this is correlated with the larger size of 

 certain blastomeres. It is thus in contradiction to Balfour's (80) law, 

 that a greater amount of yolk retards the rapidity of cleavage. Kofoid 

 (94) has suggested that this difference in rapidity of cleavage is due to 

 the greater absolute amount of protoplasm in the larger cells. 



In general this rhythm can be recognized for six or seven cleavages 

 in Planocera, although, as stated, there are often exceptions to it. 



The cells of the first quartet are only slightly smaller than the cells 

 of the basal quartet (fig. 8). The inequality, while sufficient to be 

 easily recognized, is not so great in this species as in Discocoelis and 

 most other polyclads. Girard (54) has described the cleavage of 

 Planocera elliptica as total and equal. In all others so far as known it 

 is unequal. All the cells of the first quartet are approximately equal 

 in size (fig. 8), notwithstanding the inequality of the cells from which 

 they arose. Of these cells in Discocoelis Lang says (p. 331) : " Anschein- 

 end sind die vier Ur-Ectodermzellen [first quartet] gleich gross ; es ist 

 aber sehr leicht moglich, dass sie in Wirklichkeit jihnliche Grossen- 

 unterschiede zeigen, wie die vier grossen Blastomeren .... well sie 

 bei ihren weiteren Theilungen ganz genau demselben Rhythmus folgen, 

 wie die vier grossen Blastomeren." 



Fourth Cleavage — Eight to Sixteen Cells. 



The next cleavage is initiated by the division of the largest of the 

 macromeres, ID. This division is followed very closely by the cleavage 

 of IB (PI. XXXVI, fig. 9). Before the daughter cells 2d and 2b have 

 been completely separated spindles appear in the two lateral cells lA 

 and IC The formation of this second quartet takes place in a Iseotropic 

 direction (fig. 9). By the separation of these cells, the cells of the 

 first quartet are pushed towards the left, thus in the opposite direction 

 from which they were given off. While the macromeres are dividing 

 the fii'st quartet begins to divide, also in a Iseotropic direction (fig. 9). 

 These cells divide very nearly synchronously, but in very nearly all cases 

 the spindle of the posterior cell Id is further advanced than that of the 

 others (fig. 9). The result of this division of the first quartet is eight 

 cells very nearly equal in size (fig. 10). When the divisions are 

 completed the upper or apical cells have rotated almost 45 degrees to 

 the left, so that they now occupy a position very nearly over their 

 respective macromeres (comp. fig. 12). They would no doubt occupy 

 such a position were it not for the inequality of the macromeres, which 



