282 MEAD. [Vol. XIII. 



worked out — Clepsine, Rhynchelmis, Nereis Dunimerillii, 

 Nereis limbata, and Amphitrite, and also in molluscs (cf. 

 Lillie,3 p. 25). Whether the individual cells of the somatic 

 plate (descendants of the somatoblasts) are homologous in 

 different animals cannot be discussed profitably until we 

 have more data. The only products of the somatoblast 

 whose exact origin and destiny are known are, I believe, the 

 teloblasts (neuroblasts and nephroblasts) in Clepsine and the 

 paratroch cells in Amphitrite. Wilson considers certain of its 

 products in Nereis to be without doubt the homologues of 

 the neuro-nephroblasts in Clepsine; but it seems to me that 

 the ground for such an homology is extremely insecure 

 (p. 250). 



The relationships of the other cells can be more clearly 

 understood by comparing their more highly differentiated 

 products at the 32 or at the 64-cell stage. 



^2-cell stage, the secondary trochoblasts. — In annelids with 

 a free-swimming trochophore, Lepidonotiis, Amphitrite, Cly- 

 menella, Nereis limbata, etc., the sixteen cells divide at almost 

 the same time into thirty-two cells ; they always have the same 

 relative position, forming eight quartettes which alternate 

 from pole to pole. The upper sixteen cells constitute the 

 umbrella, the lower sixteen the subumbrella. «^ b'^, c^, and d'^, 

 each divides obliquely into two cells, and it is the destiny of the 

 upper products of the first three («^"', V"'^, and c^'^) which now 

 interests us. The cleavage of these secondary trochoblasts 

 has been followed in detail only in Amphitrite and Clymenella, 

 but in these forms their behavior is similar to an extraordinary 

 degree. The divisions are the same in all three quadrants : each 

 secondary trochoblast divides into four — three lajrge cells and 

 one which is very minute. The former soon acquire cilia and 

 form part of the prototroch, while the minute dell does not 

 become ciliated, and later divides again. A more impressive 

 example of precise similarity in otigin and destiny of cleavage 

 cells in very late stages could hardly be imagined. 



The cleavage of Scolecolepis, as far as it has been followed, 

 sustains this homology of the secondary (as of the primary) 

 trochoblasts, for the three secondary trochoblasts {a"'"', b'^''^, and 



