56 CONK LIN. [Vol. XIII. 



border becomes pointed, as shown in Fig. 14. This it is seen 

 that the shape of the cell depends, in part at least, upon the 

 position which it holds, i.e., the outlines of the cell are the 

 result of the pressiire to which it is subjected. These micro- 

 meres at first meet each other in a point immediately under 

 the polar bodies, though afterward, as the result of pressure, 

 two of them may meet in a line or secondary polar furrow, as 

 shown in Fig. 17. This secondary polar furrow is not a part 

 of the original polar furrow, but is a new feature caused by the 

 shifting of the cells of the first group of micromeres after they 

 have been formed. Moreover, it bears no constant relation to 

 the original polar furrow; in Figs. 17, 29, 31, 33, 42, 44, 46 

 this secondary polar furrow is almost parallel with the original 

 one ; in Figs. 32, 35, 36, 38, 41, 49, 64 it is nearly at right 

 angles to it, and there is evidence that in the same o.^^ it may 

 change its relations at different periods. Among small cells 

 very actively dividing polar furrows, or rather pressure surfaces, 

 do not long preserve definite axial relations. The original polar 

 furrow preserves its fixed position because it lies between 

 macromeres, which in spite of numerous divisions still remain 

 very large ; the position of the polar furrow could not here be 

 changed without profound changes in the positions of all the 

 other cells and in the shape of the whole &gg. 



In Crepidula the dorsal portion of the polar furrow does not 

 lie between any of the ectoblast cells, since in all cases the cells 

 of the first quartette meet in a point when first formed ; the 

 polar furrow lies wholly and entirely between the macromeres, 

 and its dorsal portion can be seen just beneath the cap of 

 ectoblast cells. 



Kofoid ('95) has found that in Limax both the dorsal and 

 ventral polar furrows preserve their identity even up to an 

 advanced stage of the cleavage, and here the axial relations of 

 both furrows are also preserved. Kofoid says (p. 55): "With 

 the completion of the sixteen-cell stage and the fifth genera- 

 tion, the dorsal and ventral cross furrows are restored to the 

 conditions of the four-cell stage, i.e., they cross each other at 

 approximately right angles. A similar restoration to the con- 

 ditions of the four-cell stage occurs in Nereis; also in Umbrella 



