28 CONKLIN. [Vol. XIII. 



take no part in this shifting, and the ectoblast on the postero- 

 ventral side of the ovum moves in an opposite direction, i.e., 

 forward on the ventral side. There is thus a stationary point 

 in the ectoblast on the posterior side of the &gg, in front of 

 which the ectoblast cells are shoved forward, both on the 

 dorsal and ventral sides. This stationary point is just ventral 

 to the region of the future shell gland, and probably corre- 

 sponds to the posterior growing-point of the annelids. 



Organs formed from First Quai^tette. — Those cells of the first 

 quartette which lie posterior to the lateral arms of the cross, 

 grow very large and become covered by fine cilia, which pro- 

 trude through a thin cuticula. These are the cells of the 

 posterior cell plate, and they form the principal part of the 

 walls of a large head vesicle. 



The four central or apical cells give rise to an apical sense organ. 

 Each cerebral ganglion is formed at least in part from the cells 

 of the " rosette series " lying on each side of the mid line and 

 between the bases of the anterior and lateral arms of the cross ; 

 secondarily the ganglia become connected with the apical 

 organ and with the pedal ganglia and otocysts. The eyes are 

 formed in connection with the cerebral ganglia. All the turret 

 cells lie in the velum, and at least the two anterior ones con- 

 tribute to the formation of the first velar row. The cells of 

 the lateral arms of the cross divide repeatedly, and some of 

 them form part of the velum. The anterior arm forms a plate 

 of seven large cells reaching from the apical cells to the velum. 



Organs formed from the Second and Third Quartettes. — A 

 portion of the velum completely surrounds the first quartette. 

 That part of the first velar row which lies at the ends of the 

 arms of the cross, is formed from the second quartette ; the 

 intervening portions, on the anterior side, come from the first 

 quartette (turret cells). The velum is many cell-rows wide, and 

 consists of a preoral and postoral ridge bearing long flagellae 

 and an adoral ciliated groove lying between the two. Dorsally 

 the velum divides into anterior and posterior branches, which 

 are separated by the posterior turrets and the other cells of 

 the posterior plate. The anterior branch runs in on each side 

 toward the apex, and ends on each side of the apical organ ; 



