Il6 CONK LIN. [Vol. XIII. 



in the same direction is a violation of the rule of alternating 

 cleavages; but to just the extent that this cleavage ceases to be 

 perfectly spiral, it becomes bilateral. This division of the tip 

 cells is an equal one, but the posterior tip cells are much larger 

 than any of the others. At a later stage. Fig. 53, the two tip 

 cells of the posterior arm again divide, and this time also in the 

 same direction as the preceding cleavage. While the law of 

 alternation is thereby violated, bilateral and teloblastic cleavages 

 are established. 



Very soon after this division of the tip cells, the lower right 

 cell (2a'-^% etc.) in each quadrant divides in a laeotropic direc- 

 tion into two cells (2a'-"^' and 2a'-='-^-^), of which the upper is 

 somewhat the larger; and immediately following this division the 

 right middle cell (2a'-^-S etc.) divides in the same direction into 

 two cells (2a'-^-'-^ and 2a'-^'-^, etc.), of which the upper one is 

 slightly the smaller. Figs. 46, 47. Coincident with this last 

 division the left upper and middle cells in each quadrant (2a^-'-' 

 and 2a2'-^, etc.) divide in a horizontal direction into approxi- 

 mately equal products, Figs. 46, 47, and Diagram 8. 



Of the six cells described on the previous page and shown in 

 Diagram 7, all have now subdivided except the lower left one 

 in each quadrant. There are thus formed eleven cells of the 

 second quartette in each quadrant, or forty-four cells in all. 



The divisions of this quartette have been in pairs both as to 

 time and direction of cleavage. The table on the following 

 page summarizes the history of this quartette. 



Beyond this stage I have not traced the lineage of the entire 

 quartette. I believe, however, that the progeny of some of 

 these cells can be recognized at a more advanced stage, though 

 I have not seen the spindles by which they are formed. In 

 Fig. 56, for example, each of the two terminal cells in the right 

 and left arms of the cross (2a'-'-'-^ and 2c'-'"-% etc.) has divided 

 in a bilateral way, forming a row of four cells running around 

 the end of each arm. The terminal cells of the anterior arm are 

 very small and apparently do not divide ; their peculiar history 

 in C. plana has been described (p. 89). In Fig. 50 the cells 

 lying just anterior to the tip cells are probably 2b'-^-^-'' and 

 2bi.2.2.i.2^ and, accordingly, the large cell lying peripherally to 



