CLEAVAGE IN ANNELIDS. 



99 



and these are followed by the formation of a fourth quartette, 

 of which three of the members become mainly entoderm, while 

 a fourth contains the mesoderm. This fourth micromere, aris- 

 ing from the posterior cell jZ^, is of the same size as the other 

 members of the quartette, and at the sixty-four-cell stage is only 

 distinguishable from them by its position. But, while the others 

 enter into the formation of the entoderm, this divides bilaterally 

 at the surface, and, after dividing off small entoderm cells (the 

 "secondary mesoblast " of Wilson), each divides to form the 

 mesoblast of the corresponding side. This cell, ^d, is separated 

 from jD at the forty-cell stage, and the sixty-four-cell stage, 

 which is actual rather than ideal in this animal, is completed 

 by the division of the primary trochoblasts, the division of the 

 intermediate cells lying between the arms of the -cross, and by 

 the division of the second quartette of micromeres, now two in 

 number in each quadrant. The latter division presents some 

 features of importance. In each of three quadrants the ecto- 

 meres divide in such a way that, of the resulting four cells, two 

 smaller are above, and two larger are below. In the remaining 

 quadrant, on the other hand, while one of the cells divides like 

 those of the other quadrants, the other divides in a different 

 way, and there is given off a large cell above, and a very small 

 cell below, which has a deeply staining nucleus, and is an 

 important landmark in, the further study of the embryo. This 

 small cell, as later study shows, lies directly over the micro- 

 mere ^d, and since the descendants of 2d are conventionally 

 distinguished by the term X, this cell {2d2, 2) would be in this 

 system of nomenclature Xi, 2. Exactly this cell has been found 

 in a number of other annelids, including those with the most 

 unequal cleavage, and in Unio^ and Crepidiila, among the mol- 

 luscs. I have followed the living &g^ up to the stage where this 

 cell appears, and find that the " polar furrow" retains its posi- 

 tion, at least, until then — a position which agrees with the 

 position assumed in AinpJiitrite and Areiiicola? By the aid of 

 this cell, X\, 2, and the mesoblast cell, which lies immediately 

 under it, orientation of the embryo is complete. 



1 I.illie, "Embryology of the Unionidae,"y(??/;'«. of Morph., vol. x. 

 '^ Child, " Cleavage of Arenicola cristata," Zobl. Bulletin, vol. i, No. 2. 



