The Development of Ischnochiton. 589 



the functional prototroch : for tins reason I have designated them the 

 accessory trochoblasts. Each of the primary trochoblasts at 

 the 63 cell stage divides in a leiotropic direction giving rise to a 

 group of four cells in each quadrant, occupying about the same 

 position held by the parent cells. Somewhat later the accessory cells 

 divide in a dexiotropic direction forming two cells in each quadrant. 

 These occupy the space between their parent cells (/«^ ^-^ etc.), 

 which now correspond to the basal cells of the cross in Gastropods, 

 and the primary trochoblasts. 



Meanwhile changes are going on in the second quartette by 

 means of which certain cells are formed that also enter into the 

 formation of the functional prototroch. When the embryo consists of 

 32 blastomeres each of the cells of the second quartette by a dexio- 

 tropic division cleaves into two cells; the upper right hand one in 

 the 63 cell stage dividing again forms the tip cell of the cross. 



In quadrants A, B and C each of these tip cells divides by an 

 almost meridional cleavage into two equal products which ultimately 

 becoming ciliated enter into the functional velum. In quadrant D on 

 the other hand the same division of the tip cell occurs, yet the cells, 

 before the prototroch becomes functional , are drawn below the level 

 of that organ and losing their original function they enter into the 

 formation of the ventral plate. In this way a posterior gap occurs 

 similar in all respects to the prototrochal gap of Annelids. 



All the cells of the prototroch are now formed and comprise eight 

 accessory, sixteen primary and six secondary trochoblasts, thirty cells 

 in all. As will be seen, two cells, products of the median cell of the 

 cross, becoming ciliated aid in filling the posterior gap , but their 

 function is secondarily acquired and ought not, strictly speaking, to 

 be included in the primitive velum. 



In the incipient stages of its formation the prototroch consists of 

 an irregular band of cells encircling the embryo. The tip cells lie 

 below the level of the future velum, and the remaining cells are also 

 somewhat out of line. All the cells are bold and round in outline and 

 their exposed surfaces are apparently greater than at any period 

 until their degeneration sets in. Nevertheless very shortly after its 

 formation, in fact even before it is fully formed, the prototroch com- 

 mences to assume those features which characterize the later stages 

 of its history, and certain movements of a migratory character are 

 met with among the cells which work out decided changes in the re- 

 lation and position of the velum. Attention has already been called 



