No. 3.] THE CELL-LINEAGE OF NEREIS. 391 



group (c^, d^) are budded forth from C and D, respectively 

 as shown in Figs. 20, 21, 22. The embryo now consists of 

 twenty-three cells, of which two are median {X, b'^), ten are on 

 the left side, and eleven on the right side. A study of the 

 position of the spindles shows that the two new micromeres 

 are budded off in a right-handed spiral like the first set. (The 

 nuclear stars and spindles lie, of course, in the granular portion 

 of the macromeres.) 



Thh'd, the two anterior micromeres of the third group {cfi, b^) 

 are budded forth, in a right-handed spiral, from A and B respec- 

 tively. At the same time the four trochoblasts divide in a 

 meridional plane (Fig. 23), so that the girdle (colored blue) now 

 consists of twelve cells (twenty-nine-celled stage). 



Fourth, the three smaller micromeres of the second group 

 {a^, b"^, c^) divide approximately in a meridional plane (somewhat 

 oblique), thus completing the thirty-two-celled stage (Figs. 24, 

 25, 26; the last two, however, are already in transition to the 

 thirty-six-celled stage). 



A careful study of the embryo through these changes shows 

 that all of the cell-divisions conform to the spiral type. This is 

 at once apparent in the divisions of the four macromeres and of 

 the four primary micromeres. It is also easily seen in the divis- 

 ions of the secondary micromeres {a^, b'^, c^, X). Each of them 

 divides somewhat obliquely {cf. Figs. 25, 26, 33), so that one of 

 the cells lies somewhat lower than the other, and in most cases 

 the lower cell is obviously smaller than the upper. The differ- 

 ence in size is very great in the case of X and x^, but is much 

 less in the case of the others {a^-'^, a^-'^, Fig. 33). [In the speci- 

 men shown in Figs. 25, 26, on the other hand, there is no appre- 

 ciable difference in size, but I have never seen a case in which 

 the upper cell is the smaller.] If this group of cells be followed 

 around the embryo from right to left (against the hands of a 

 watch), the upper (larger) cell always comes first ; i.e. the first 

 division of the second group of micromeres takes place in a 

 left-handed spiral, like the second division of the Jirst set of 

 inic7^onieres. 



The division of the trochoblasts, as far as I can determine, 

 is meridional and equal ; i.e. of a true radial type, of which the 

 spiral type is a modification. 



