No. 2.] DEVELOPMENT OF MARINE ANNELIDS. 28 1 



are conspicuous for their diminutive size and their tardiness in 

 dividing, — just what we should expect of trochoblasts in a 

 suppressed trochophore, if the homology could be extended to 

 this form. Unfortunately, the further history of these cells is 

 unknown. 



In Nereis, according to Wilson,^ only twelve of the products 

 of the trochoblasts enter the prototroch, so that the (primary) 

 prototroch consists of four groups of three cells each instead of 

 four groups oifoiir cells each, as in the case of Amphitrite and 

 Clymenella. Thus the homology would appear to be somewhat 

 imperfect, but it is sustained by a closer comparison of the 

 behavior of the trochoblasts in Nereis with those in Lepidonotus, 

 Amphitrite, and Clymenella. In each, the four trochoblasts arise 

 in the same manner and divide obliquely to the right. In the 

 first three annelids the eight resulting cells again divide 

 obliquely (to the left), while in Nereis, according to Wilson, 

 the direction alternates, one cell dividing horizontally, the next 

 vertically, and so on around the egg. It will be readily 

 admitted that Lepidonotus and Amphitrite have a more typical 

 cleavage than Nereis, for they have less yolk and the cleavage is 

 more regular. If we compare the figures of Nereis with those 

 of the more regular forms, the so-called horizontal and vertical 

 cleavages show at least a reminiscence of obliquity. In Nereis 

 the four products of the trochoblasts which lie nearest the ani- 

 mal pole are said not to enter the prototroch ; in Amphitrite 

 and Clymenella these cells at first have a similar position, but 

 later certainly form part of the prototroch. In Nereis, further- 

 more, the four cells in question have not been seen to divide 

 again, and we do not know how the prototroch is completed. 



I think these facts warrant the assumption that the history 

 of the trochoblasts is the same in Nereis as in the other forms. 



In ChcBtoptenis the trochoblasts divide into sixteen cells in 

 the regular way, but some of them divide again, — a fact which 

 is significant, because Chcetoptenis is said to have no prototroch. 



Sojnatoblasi. — ■ One other cell in the i6-cell stage, the so- 

 matoblast {d^), is already distinguished from the others by the 

 fact that it contains the anlage of the whole or the greater 

 part of the trunk ectoderm in all annelids where it has been 



