THE CELL ORIGIN OF THE PROTOTROCH. 137 



For the sake of convenience in comparing the cell-origin of 

 the prototroch in the various annelids and gasteropods in which 

 it has been investigated, we assumed the annelid Amphitrite as 

 the type. In this form the prototroch arises from two sources : 

 (a) Four cells, primary trochoblasts, one belonging to each 

 quadrant of the umbrella hemisphere, give rise to a functional 

 " primary prototroch," which is for a time the only ciliated 

 portion of the larva. The four cells in question are formed 

 at the i6-cell stage, (b) Three cells, secondary trochoblasts, 

 belonging to the right, left, and ventral quadrants of the subum- 

 brella hemisphere, give rise, later, to the rest of the prototroch. 

 These three cells are formed at the 32-cell stage. The whole 

 prototroch is at first interrupted in the mid-dorsal region, but 

 ultimately grows together. 



Almost all of the positive evidence furnished by the compari- 

 sons which we have drawn points, I believe, to the conclusion, 

 first, that the component cells of the prototroch are homologous 

 in the various types studied, in the same sense as are the com- 

 ponent bones in the skeleton of the vertebrate limb. If this 

 conclusion is correct, the homology of the whole prototroch is 

 placed on a firmer basis, and the evidence of the relationship 

 of the two diverse groups of the Trochozoa annelids and 

 mollusks is likewise strengthened. As a corollary of this 

 conclusion, it would appear that, since certain cells form the 

 same organ under environmental conditions which are unlike, the 

 environmental factors do not have the importance in directly 

 shaping development which is sometimes claimed for them. 



Some of the evidence, however, seems to controvert this 

 conclusion. I refer particularly to the discrepancies between 

 Wilson's account of Nereis limbata and all other accounts in 

 regard to the destiny of four cells, and to that between Heath's 

 account of IscJmochiton and all the other forms in regard to the 

 destiny of four or more cells. 



Considering the practical difficulties, we may say with regard 

 to the value of different observations that positive evidence that 

 a certain cell enters the prototroch should have more weight 

 than the negative evidence that it does not, unless a different 

 destiny for the cell is established in the latter instance. 



