1904.] NATURAL SCIENCES OF PHILADELPHIA. 697 



and larger than the anterior pair la and lb which also arc equal. 

 This size relation between the members of the first quartette occurs 

 in Nereis (Wilson, 1892), Amphitrite and Clymenella (Mead, 1897), in 

 Arenicola (Child, 1900), and, as the figures appear to indicate, in Capi- 

 tella (Eisig, 1898), although Eisig states (p. 7) that the micromeres 

 are "unter sich aber annaherend glcich grosscn Zcllen." Among the 

 Mollusca this size relation seems not to appear at all. 



The next cleavage, the fourth, is inaugurated by the second division 

 of ID. In fig. 8 this macromere is seen to be already in the pro- 

 phase of division, and while the other cells in the ovum are preparing 

 for division ID separates by a leiotropic division into the relatively 

 enormous cell 2d and the macromere 2D, which has now been reduced to 

 the size of its fellows (figs. 9 and 10). 2d, the "first somatoblast" of 

 von Wistinghausen (1891), which I shall label X, following Goette and 

 most recent writers on the cell-lineage of annelids, lies in the second 

 cleavage furrow slightly to the left of the mid-line. This cell is a most 

 valuable aid to orientation, owing to its large size and fixed position. 

 In this last division the macromere 2D, the smaller division product, 

 becomes displaced, being crowded downwards so as to be almost directly 

 below lA (figs. 9 and 10). A similar displacement of 2D occurs also 

 in Unto (Lillie, 1895), Dreissensia (Meisenheimer, 1901), Capitella 

 (Eisig, 1898), and probably whenever the cell 2d greatly exceeds in 

 size its parent macromere. 2D subsequently returns to the level of 

 the other macromeres. 



At about the time 2d is produced the first quartette undergoes a 

 rotation in an anti-clockwise direction, so that each micromere comes 

 to lie exactly over the macromere from which it arose (cf. figs. 8 and 

 10). This rotation is undoubtedly brought about by the division of 

 ID to form 2d, the latter cell being pushed backward and upward in a 

 leiotropic direction. In Crepidula (Conklin, 1897), Arenicola (Child, 

 1900) and in other forms a similar rotation is brought about through 

 the formation of the cells of the second quartette. 



The micromeres Id and Ic next divide, the spindles being inclined in 

 a leiotropic direction. Through these divisions there are budded off 

 from Id and Ic, on their peripheral sides, small cells of equal size, 

 Id- and Ic^, kP being formed first (figs. 9 and 10). These cells, though 

 formed by a truly spiral division, come to lie laterally to their parent 

 cells and overhung by them (figs. 10, 11, and 12). A little later la and 

 lb follow in a similar division, the cells la^ and Ib^ being equal in size 

 and somewhat smaller than kP and Ic^, and come to lie in the furrow 

 below the micromeres la^ and 11)^ and to the right of them. These 

 45 



