718 PROCEEDIXGS OF THE ACADEMY OF [Oct., 



(2) 2a, 26 and 2c and the Third Quartette. 



The divisions of the cells 2a, 2b and 2c up to an advanced stage have 

 been described in dealing with the history of the prototroch. There is 

 left but one division to record, which I have observed only in 2a^-^ and 

 2c^.-^. This division is shown in figs. 34 and 35. The plane of the 

 division is nearly equatorial, but inclined slightly in a leiotropic 

 direction in the C quadrant, in a dexiotropic direction in the A quad- 

 rant. Of the two products the lower is somewhat the smaller. I have 

 never witnessed a corresponding division in 2b- -^j but it very probabl}' 

 occurs. The position and small size of this cell make it difficult to 

 study in the later stages. The two products of 2a^- and 2c^-^ respec- 

 tively are quite conspicuous up to an advanced stage, although they 

 probably divide into smaller cells just before the blastopore closes. 



The origin of the third quartette has been dealt with in the descrip- 

 tion of the cleavages. 3d is the largest cell in this quartette, 3b the 

 smallest, while 3a and 3c are intermediate in size. At the 40-cell stage 

 3d divides equally and leiotropically. The products of this division 

 are shown in fig. 22. A little later 3c divides, but in quite a different 

 manner, budding off a small cell toward the animal pole. The plane 

 of the division is equatorial (fig. 26). Next 3a also buds off a cell 

 toward the animal pole, the spindle being nearly vertical, though the 

 position of the products indicates a leiotropic division. 3b divides 

 some time after 3a, and here the spindle is decidedly leiotropic (fig. 34). 

 It is interesting to note that both 3b and 2b, in their respective quar- 

 tettes, are the most conservative in retaining the primitive direction 

 of their divisions. Wliile 3a is in division, spindles are seen in 3d^ and 

 3d^ (fig. 27). These cells bud off tiny cells toward the animal pole (fig. 

 32) . Of these latter that derived from 3d^ is the smaller. The spindles 

 for this division are nearly vertical, but sufficiently inclined to make the 

 division leiotropic, in direct violation to the law of alternating cleav- 

 ages. Even before 3b has completed its first division 3c has budded 

 off another cell toward the animal pole, the spindle being inclined 

 dexiotropically, as it should be according to the law of alternating 

 cleavages (fig. 34). With this division, or rather the one which follows 

 it, i.e., that of 3b, I have ceased to follow the cell-lineage of the entire 

 egg. 



In fig. 53 the position of the cells 2a2-2i, 2a- -2-, 2c^--\ and 2c2-2-2 is 

 clearly seen. Each pair of cells form a portion of the lateral margin 

 of the blastopore; 2a^-^-^ and 2c^-^-^, which formerly were above, that 

 is, to the animal pole side of 2a--- and 2c---, now lie anterior to them. 

 They still, however, maintain their relative position in the ectoderm, 



