1904.] NATURAL SCIENCIOS OF PHILADELPHIA. 715 



unequal parts, the left product being the greater, and occupying a 

 median position in the cleft between X and X. The cell x----^ is now 

 nearly equal to x.^'^-\ and their next two divisions (figs. 47 and 49) 

 are bilaterally symmetrical. The same may be said of the small cells 

 x^-^-^ and x^-^ (figs. 41 to 48). While I have not followed the history 

 of these latter cells further, it is extremely probable that their divisions 

 are also bilaterally symmetrical. There is here then an instance of a 

 symmetrical arrangement of cells derived by asymmetrical divisions, 

 examples of which are found in the annelid and molluscan crosses, and 

 in the X group of Ani'phitrite (Mead, 1897) and Arenicola (Child, 1900), 

 and probably also in many other forms. While x^-^-^-^ and x---^ are 

 almost equal, x^-^-^ is really somewhat the smaller of the two; and in 

 this connection it is of interest to remember that on this side, the left, 

 next to x^--\ lies the largest member of the third quartette, viz., 3d. 

 Returning to figs. 44 and 45, the cells x^-^-^ and x--^ are seen to have each 

 given off a minute cell toward the vegetal pole. These cells are also 

 shown in figs. 38 and 39. 



The products of the bilateral division of X, called by Wilson (1892) 

 the "posterior proteleblasts," and by Child (1900) the "posterior stem 

 cells," undergo a series of bilateral divisions. The first is shown in 

 figs. 45-47, X* and x^. These cells have pushed forward x'-^-'-^ and 

 -j.2.i.i_ 'Y]^Q qqY[s formed by the next division of the posterior stem 

 cells come to lie together near the median plane, and are also budded 

 oflf toward the animal pole. Meanwhile x^-^ has been divided into equal 

 parts by a meridional plane, and these three cells assume a symmetrical 

 arrangement (fig. 48). I regret that I have not followed the division 

 of the members of the X group beyond this point. Figs. 53 and 55 

 show what is probably the next division of the posterior stem cells, 

 by means of which two large cells, x*'-x'^, are separated off ventrally 

 and laterally. After the closure of the blastopore the posterior stem 

 cells each undergo a further subdivision into two cells by a meridional 

 cleavage. The four cells thus formed resemble in appearance the 

 "posterior tcloblasts" of Nereis (Wilson, 1892). 



The lineage of those cells which lie on the vegetal pole side of the 

 posterior stem cells was followed as far as is indicated in the figures, but 

 a consideration of their cleavages is of little value, inasmuch as there 

 is no sign of a paratroch at the time when the blastopore closes, and 

 the cells on the ventral side of the embryo have by this time become 

 so small and thin as to be very difficult to distinguish satisfactorily. 



There are two points in connection with the cleavage of X which 

 require special mention. The first of these concerns the first four 



