716 PROCEEDINGS OF THE ACADEMY OF [Oct., 



cleavages of X. These cleavages are essentially the same as those of X 

 in Amphitrite and Clymenella (Mead, 1897) and Arenicola (Child, 1900). 

 This similarity extends not only to the direction of the cleavages, 

 but also to their products, which bear to one another a very similar 

 size relation. This striking resemblance cannot be ascribed to the 

 effect of alternating cleavages, since at the third cleavage this law is 

 violated in x*\ at the fourth in X. The divisions of X in Dinophilus 

 and Nereis (Wilson, 1892) differ no more than do the corresponding 

 divisions in Nereis and other annelids {Amphitrite, etc.). These differ- 

 ences are, that in Nereis x^ is formed exactly in the dorsal mid-line, 

 and that the division of x^ is delayed, and is nearly equal when it occurs. 

 In all the Polychata whose cytogeny is known (except Podarke) bilat- 

 erality appears in X at the fourth cleavage. It may then be said of 

 Dinophilus that hilaterality appears in the cell X in the same cell genera- 

 tion as in all polychcetous annelids investigated having the unequal type 

 of cleavage, and appears in x^ at the same cleavage as in at least three 

 polychcetous annelids. 



The second point concerns the arrangement of the cells of the X 

 group. In Nereis the main body of the descendants of X are so dis- 

 tributed that they come to lie on the vegetal pole side of the stem cells, 

 the latter remaining near the prototroch. In the other Polychaeta, up 

 to the time of the closure of the blastopore, the descendants of X are 

 uniformly distributed about the posterior stem cells, so that up to a 

 late stage the latter occupy a central position with regard to their 

 products. In Dinophilus, on the other hand, the greater part of the 

 descendants of X, up to the time w^hen the blastopore is closed, are 

 distributed dorsal and lateral to the posterior stem cells. This pecu- 

 liarity is related to the peculiar shifting of areas, which has already 

 been briefly described in the chapter on the later development. This 

 distribution is not due entirely to a difference in the direction of the 

 division of X — though in Nereis two cells, x* and x^ are budded off 

 toward the vegetal pole — but to a shifting of the cells among them- 

 selves. For example, in Amphitrite the cells x^ and x- shift to the 

 vegetal pole side of X, while in Dinophilus the same cells always retain 

 a lateral position. 



The further history of this group, as far as I have been able to trace it, 

 is as follows. At a time near the closure of the blastopore, as shown in 

 fig. 55 and text fig. IV, E, the first quartette has been shifted forward 

 through nearly 90 degrees, being pushed forward through the forma- 

 tion of x^-x'*, x^-x'^, etc., until the descendants of these cells cover the 

 dorsal surface of the embryo up to the limits of the first quartette and 



