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TREADWELL. 



[Vol. XVII. 



cells, iai.2, ibi.2, IC1.2, idi.2 ; and {c), a division of the third 

 group of micromeres. See PI. XXXVI, Fig. 12 ; PI. XXXVII, 

 Fig. 14, where the spindles of all these divisions are shown. 

 J. The second division of the second group of ectomeres 

 (PI. XXXVII, Fig. 24). This is a most important division, for 

 by it arises the first indication of a difference between the quad- 

 rants. It marks, in other words, the first appearance of bilat- 

 eral symmetry. In three of the quadrants the mode of division 

 is that shown in PI. XXXVIII, Fig. 25 (2bi.i, 2bi.2, 2b2.i, 

 2b2.2, etc.). In the fourth quadrant the case is different. 

 The dextral cell, 2di, has divided, as in the other quadrants, 



with a smaller cell above. The 

 sinistral cell, on the other hand, 

 has divided in precisely the op- 

 posite way. The larger cell is 

 above, and the smaller cell, 

 which is very much smaller 

 than any products of the divi- 

 sion in the other quadrants, is 

 below. This cell lies a little 

 to the right of the second 

 cleavage plane, and nearly 

 over one of the fourth group 

 of micromeres. It can easily 

 be distinguished by its small 

 size and deeply staining nu- 

 cleus (PI. XXXVIII, Fig. 26). 

 Later study shows that the 

 "4" micromere, near which it lies, contains the mesoderm. 

 Orientation is now complete, and the quadrant in which the 

 cell in question occurs is the " D " quadrant. A similar cell, 

 occupying a similar position, has been described by Mead (No. 

 22), Lillie (No. 21), and Conklin (No. 5, a) in other annelids 

 and mollusks. The polar furrow retains its original direction 

 up to the time when this cell appears, and this enables us to 

 orient the early cleavage stages. 



At about the 40-cell stage strands of protoplasm can be 

 plainly seen reaching across the cleavage cavity, running from 



Fig. 4. — Optical section of stage shown in Fig. 

 16, PI. XXXVII. In this and later draw- 

 ings the nuclei of the prototroch cells are 

 stippled. 



