1904.] NATURAL SCIENCES OF PHILADELPHIA. 705 



VI. — History of the First Quartette. 

 (1) The Cross and the Intermediate Girdle Cells. 



At the time of the formation of the mesoblast cell 4d, the first quar- 

 tette consists of twelve cells — four stem cells, four intermediate girdle 

 cells and four trochoblasts (fig. 14). According to the ideal scheme 

 the trochoblasts should by this time have divided once, but their rate 

 of division is very greatly retarded, obviously in correlation with the 

 comparatively late period at which they become functional. 



The stem cells are the next to divide. Even before 4d is formed, the 

 posterior stem cells (Id^-^ and Ic^-^) are preparing for a leiotropic 

 cleavage (fig. 14), These divide and la^-^ and Ib^-^ soon follow; the 

 upper (left) products resulting from the division are four small cells 

 of equal size, forming a quatrefoil at the animal pole, the ''rosette" 

 of Nereis (Wilson, 1892) and other annelids (fig. 20). 



While the rosette is forming, the intermediate girdle cells arc also 

 engaged in a leiotropic division which is unequal in all save the D 

 quadrant (fig. 20). In the A, B and C quadrants the right and periph- 

 eral product is a small cell, to which, in consideration of its origin and 

 probable fate, has been given the name "accessory trochoblast," a 

 term applied by Heath (1899) to cells of Ischnochiton of similar origin 

 and fate. 



While the division of the stem cells and intermediate girdle cells is 

 in progress the trochoblasts also divide (figs. 20, 21, 22 and 29). 

 Ic^ and Id^ usually divide at nearly the same time, although in fig. 14 

 Ic^ is the only trochoblast showing a spindle; la^ follows more or less 

 closely on Id- and Ic', while lb- is always delayed. The division is 

 meridional in the posterior trochoblasts ; in the anterior pair the plane 

 of division, although very nearly meridional, is nevertheless inclined 

 sufficiently to indicate a dexiotropic cleavage (fig. 29). 



Returning to the stem cells, the posterior pair of these are found to 

 be again in mitotic activity, their spindles having reached the meta- 

 phasc before the last division of Ib^"^ is fully completed (fig. 20). 

 These spindles are not spiral, but truly radial in position. Their 

 central ends lie at a higher level than their peripheral ends. 



This division marks the beginning of bilateral cleavages in the first 

 quartette, though these do not appear in the intermediate girdle cells. 

 In fig. 20 the symmetrical position of the spindles in Ic*-^ and Id^-* is 

 obscured owing to the clockwise rotation of the first quartette, caused 

 by the formation of x^ in a dexiotropic direction. In fig. 25 may be 

 seen the products of this division: of these the peripheral products 

 are slightly the larger, and are overlapped by the central products. 



