714 PROCEEDINGS OF THE ACADEMY OF [Oct., 



The two meridional cleavages of the primary trochoblasts and the 

 participation of 2a?\ 2b2-i and 2c--^ in the prototroch tend toward the 

 same end, i.e., the production of a narrow band of small cells. 2a, 2b 

 and 2c, instead of each giving off one large cell to the prototroch 

 which subdivides into four cells, covering a broad area, each lays 

 down side by side two small cells, which may then divide meridionally 

 (or nearly so) without violation of the law of alternating cleavages, and 

 thus increase the length of the prototroch, but not its breadth. 



VII. — History of the Second and Third Quartettes. 



(1) 2d ( = X). 



The origin of 2d ( = X) has already been described. The size of this 

 cell is enormous as compared with that of the other cells of the embryo. 

 This relatively enormous size recalls the conditions found among the 

 lamellibranchs, Cijclas (Stauffacher, 1893), Unto (Lillie, 1895) and 

 Dreissensia (Meisenheimer, 1901), and among annelids in Arenicola 

 (Child, 1900), and also in Clepsine (Whitman, 1878). The first two 

 divisions of this cell have already been described ; the products of these 

 divisions and the first product of the third division, x^ are shown in fig. 

 42. In this figure it is seen that x^ and x^ are not precisely symmetrical 

 with respect to X, either in size or position, since x^ is somewhat larger 

 than x^ and placed at a higher level on X. x^-^ lies precisely in the 

 ventral mid-line, x^ lies dorsally to the left, as viewed from behind, 

 and has been formed by a dexiotropic division of X. The approaching 

 division of x*-^ and x^-^ as shown in the figure is of interest, since x^-* 

 is the first cell in the embryo to violate the law of alternating cleavages, 

 inasmuch as the spindle for this division has the same direction as 

 that which formed x^-^. This reversal of the spindle in x^-^ was first 

 pointed out l)y Mead (1897) for Ampkitrite, and was also found by 

 Child (1900) in Arenicola. The products of this division and the cor- 

 responding one of x2 are shown in fig. 43. In this figure are also shown 

 the spindles for the fourth cleavage of the X group. Two points are 

 to be noted in this cleavage : (1) the division of x^ has evidently pushed 

 x^ toward the left (right in the figure), bringing it almost into the mid- 

 line ; and (2) X is dividing into bilaterally placed halves. This bilateral 

 division of X is a striking and constant feature of the unequal type of 

 cleavage among the polychsetous annelids. A similar division occurs 

 in the lamellibranchs, but appears at the fifth cleavage, instead of at 

 the fourth. All the products of the fourth cleavage of X are shown 

 in fig. 44. x^-^^ has budded off a small cell below; x^-^ has divided into 

 neariy equal parts by a meridional cleavage; while x^ has split into 



