244 MEAD. [Vol. XIII. 



pretty severe test of the influence of bilaterality of the whole 

 organism upon the cleavage in certain parts. The two subse- 

 quent cleavages are therefore of special interest, for they give 

 to the question an unequivocal answer. The next division takes 

 place with total disregard of the symmetry of the grotip, and 

 results in a new arrangement of the cells, which is sytnmetrical 

 with regard to tJie middle line of the embryo (Figs. 46-52 and 

 text Fig. IV, c, d). 



The middle cells P and P are sister cells, equal in size and 

 symmetrically placed in the group, but they divide very differ- 

 ently (text Fig. IV, c) : V divides like an impaired median cell and 

 its equal products lie one on either side of the median line of 

 the embryo; I^ divides symmetrically with IP; III'' divides 

 so that its products correspond to the (undivided) cells IP 

 and IIP. The grouping is thus rearranged, a new symmetry 

 established, and the middle line of the new group coincides 

 with that of the embryo. This is shown to be true, not only 

 by the position of the cells, but by their subsequent divisions : 

 corresponding cells divide symmetrically — bilaterally (Figs. 

 49-55 and Diagram III, d, e). 



The four larger products of the four median cells are the 

 paratrochal cells. They never divide again, and constitute 

 the perianal paratroch of the trochophore, which persists as a 

 ring of ciliated cells around the body just in front of the anus, 

 until the larva has developed five or six metameres (Figs. 53- 

 59, 61-64, light brown, text Fig. IV, c, with nuclei). The 

 smaller products of these cells lie within the arc formed by the 

 paratrochal cells and give rise to those structures only, which 

 are posterior to the paratroch, — the proctodoeum, etc. Since 

 they mark the posterior end of the larva, I have called them 

 terminal cells. 



The following facts in regard to the origin of the paratrochal 

 cells and the enclosed terminal cells, though obvious, are too 

 important to be left unmentioned : 



(l) They all arise from the somatoblast X. (2) They lie 

 at the posterior lip of the blastopore or entoderm plate. (3) At 

 first the four lie nearly in a straight row, only the tiuo dorsal 

 ones meeti7ig in the middle line. (4) Three of the fonr are 



