18G TORRE Y 



12 cells. Child ('oo) ascribes the flattening and lengthening of 

 the prototroch cells in Arenicola to their being pushed down 

 posteriorly by the praetrochal divisions. This is, no doubt, in 

 a measure true in TJialassema. 



The prototroch of the trochophore is completed (Text-Fig. 

 3, B) by the addition of three more cells from the first quartet 

 (PL I, Figs. 6 and 7) and nine from the second (PI. II, Fig. 24). 

 As has already been shown, the three cells from the first quartet 

 arise from two divisions of the a, b and c intergirdle cells (\a 

 1.2. 2.2 » i^''i.2. 2. 2» i^^i. 2. 2. 2)- These same cells also enter the 

 prototroch in Podarke. In Isclinochiton (Heath, 99) both prod- 

 ucts of the last division enter the prototroch. It is interesting 

 to note that the dextral cell, which is the smaller in IcJino- 

 chilon, is entirely rudimentary in the b quadrant in Thalassema^ 

 and also in Ampliitrite . Mead ('98) has called attention to the 

 interest in the intergirdle cell that has subsequently been found 

 to enter the prototroch in Podarke, and ** recommends the in- 

 vestigation of its destiny ... in Annelids as a fruitful problem 

 in cell-lineage." 



The cells which later enter the prototroch from the second 

 quartet are the same as in Ampliitrite, Clymenella and Arenicola 

 (2^1 1 J , 2^1 1 o , 2^1 2.1 > etc.). In Podarke the upper cells of 

 these three groups (2^^ ^ ^ , 2b^^^, ^^-i.i.i) ^^^ small and are 

 said to be pushed later entirely out of the prototrochal ring. 

 Although in Thalassema this cell is smaller than the other 

 two, there is no doubt that it here contributes to the pro- 

 totroch. All of these cells are added to the prototroch and 

 become ciliated very late in the development, as is also the 

 case in Podarke. 



To sum up : TJie complete prototroch in Thalassema consists 

 of 28 cells as against the iisnal mtmber, 2j ; the piimary proto- 

 troch is " completed'' by fonr cells from each of the a b and c 

 quadrants, of luhich one is contributed from the first quartet and 

 three from the second} 



1 Conn ('86) manifestly intiuenced by Hatschek's earlier descriptions, has erro- 

 neously described the prototroch in Thalassema as consisting of a pn^-oral and post- 

 oral row of "high prismatic cells." The yellow color of the prototroch cells in 



