706 



PROCEEDINGS OF THE ACADEMY OF 



[Oct., 



owing to the oblique position of the spindle. Meanwhile the first 

 quartette has rotated in an anti-clockwise direction, being thus restored 

 to its original position. Next la^-^- and Ib^-^-^ divide, the spindles in 

 their cells being also truly radial, with the central ends the higher. 

 The cell pattern produced by these divisions is similar to that first 

 found by Wilson (1892) in Nereis, Polymnia, Spio and Aricia, and 

 called by him "the cross." The characteristic feature of the cross is 

 the radial divisions of the stem cells. This, as has just been described, 

 is also the case in Dinophilus, but the cell pattern is marred by the 

 belated division of Ib^-^-^, which does not occur until the basal cells of 

 the posterior arms of the cross are again in division (fig. 30). The 

 cross is formed also in Amphitrite, Clijmenella and Lepidonotus (Mead, 

 A B 



I'h 



V, A, the "cross" of Nereis, copied from Wilson (1S92); B, diagram of the 

 "cross" of Dinophihis. The four cjuadrants are represented as having 



divided simultaneously. 



1897), Capitella (Eisig, 1898), Arenicola and Sternaspis (Child, 1900), 

 and Podarke (Treadwell, 1901). In Podarke the spindles are slightly 

 dexiotropic, and in Chcetopterus (Mead, 1897) they are so much so 

 that the cross is not formed at all. In Dinophilus the spindles are 

 truly radial, no trace of the spiral tjqoe having been observed. The 

 comparison of the Dinophilus cross and that of the polychatous annelid 

 Nei'eis will be made clear by reference to text fig. V, A. and B. 



The radial spindles forming the cross mark the appearance of bilat- 

 eral cleavage in the first quartette. Not only is the cross in itself a 

 symmetrical structure, but it is bilaterally symmetrical with respect 

 to the median plane of the embryo, and this is, as Wilson pointed out 

 in Nereis (1892), "an adult bilaterality foreshadowed, long before 



