THE CELL ORLGIN OF THE PROTOTROCH. 123 



globules in the earlier drawings, and by the apical tuft of 

 flagella in Figs. 14-16. The median tentacle grows out just 

 ventral to the apical tuft. The posterior end is the middle of 

 the area which becomes enclosed by the paratroch cells, and 

 here the proctodaeum is formed (Fig. 16). The "budding 

 zone " lies just in front of the paratroch, and it is by the rapid 

 growth in this region that the trunk elongates. The distance 

 between the prototroch and paratroch, therefore, continually 

 increases. 



We stated at the beginning of this paper that, inasmuch as 

 the trochophore is a highly representative larval form, the 

 embryonic origin of one of its essential organs might prove to 

 be of especial value, and the history of the origin of the proto- 

 troch in the one species which we have reviewed strengthens 

 this conjecture and brings out some additional points of in- 

 terest. 



The prototroch is differentiated as a definite functional 

 organ at a very early stage in development, when the whole 

 body consists of less than i 50 cells. It is composed of com- 

 paratively few cells (twenty-five), which are constant in num- 

 ber, in size, in relative position, and in mode of origin, in all 

 individuals of the species. The portion of the larva differen- 

 tiated as the prototroch is sharply marked off from the surround- 

 ing areas by the cell boundaries, and the cells in the immediate 

 vicinity of the prototroch do not especially resemble the latter 

 either in behavior during cleavage or in ultimate function. 



Having thus taken our bearings, we find ourselves in this 

 position. The trochophore is a larval form characteristic of a 

 large group of animals, annelids, gasteropods, rotifers, etc., and 

 the prototroch in one species, at least, arises always in the 

 same manner from certain cells through a definite process of 

 cleavage. Although the origin of the prototroch is compara- 

 tively simple, yet it is sufficiently complex to offer ample 

 opportunity for variation, and by comparing its cell-origin in 

 other animals we may hope to ascertain whether it always 

 arises from the same cleavage cells. If it does so arise, then 

 we may consider that its component cells are homologous, and, 

 of course, the homology of the prototroch itself will thereby be 



