388 WILSON. [Vol. VI. 



spindles only that in Chas a radial position. The other three 

 are oblique, and already show clearly the direction of the future 

 divisions. Figs. 8 and 9 represent in two positions an embryo 

 in the closing stages of the division, and show the increased 

 obliquity of the spindles. Figs. 10, 11, 12 are the completed 

 eight-celled stage. The pritnaiy cause of the rotation, therefore, 

 lies within the cells, though it may be heightened by external 

 causes operating at the time of division, or subsequent to it. 



It is an interesting fact that the two posterior micromeres, 

 c^ and d^, are usually (but I believe not always) formed slightly 

 earlier than the others. This is the first indication of a ten- 

 dency, henceforward more and more marked, towards an accel- 

 eration of development in the products of the two posterior 

 macromeres, which may be followed up to a late stage. This 

 fact, which has been observed in embryos of several mollusks 

 and polyclades, is especially noteworthy in the case of Nereis, 

 since the anterior of the two primary blastomeres is always the 

 first to divide (again as in some mollusks and polyclades) ; and, 

 moreover, it is the posterior region of the annelid that in later 

 stages longest retains the embryonic character. 



4. The Fourth Cleavage (Figs. 13 to 18). 



The fourth cleavage, which takes place about two and one- 

 half hours after fertilization, is of extreme importance, since of 

 the eight new blastomeres five can be identified as definite pro- 

 toblasts, each of which form a new point of departure. The 

 essential features of the cleavage are as follows : The four 

 micromeres divide unequally in a right-handed spiral, while the 

 four macromeres divide unequally in a left-handed spiral. An 

 examination of Figs. 13 to 17 will obviate the need of a detailed 

 description. The four smaller cells derived from the micro- 

 meres {a^-^, b,^-^ c,^-'^ d^-^) (blue) are the parent-cells of the proto- 

 troch, and hence viay be tenned trochoblasts. 



The four new cells derived from the macromeres {d^, b"^, c^, d^ 

 or X) may be called the second group of micromeres. Three 

 of them {a^, b'^, c"^) are of nearly equal size ; the fourth {d"^ or X, 

 colored brown throughout the plates) is much larger, and will 

 henceforward be termed Xhe, first somatoblast. The three smaller 

 micromeres give rise to the stomodaeum and to a portion of the 



