No. 2.] DEVELOPMENT OF MARINE ANNELIDS. 287 



of the prototroch, and contribute to the ectoderm of the sub- 

 umbrella (p. 239). 



The cells of the prototroch have already been considered, and 

 this brings us to the lower hemisphere, or subumbrella. While 

 the somatic plate as a whole has a similar origin and fate in the 

 various annelids, we have not data enough to determine whether 

 the homology may be extended to the individual cells. It 

 seems probable that the "neuroblasts" and " nephroblasts " of 

 Clepsme, Rhynchelmis, and Lumbricus are homologous. 



Wilson believes, moreover, that the proteloblasts in Nereis, 

 are the homologues of the netiro-nephroblasts in Clepsine, but I 

 find nothing in Amphitrite to support this view. 



The early divisions of the cells of the somatic plate have a 

 striking similarity in the various forms. One cell in particular is 

 conspicuous for its similarity in Nereis, Amphitrite, Clymenella, 

 and Unio. It lies in the lower right-hand corner of the somatic 

 plate (marked x^''^) : it gives rise to part of the proctodseum in 

 Amphitrite. A comparison of the exact origin of the paratroch 

 in the various forms would be of greatest interest. 



The secondary trochoblasts have been discussed. As to the 

 rest of the ectoderm on the lower hemisphere, we know only 

 the general fate of groups of cells. In Nereis Wilson says that 

 the terminal cells which form the proctodseum are " certainly 

 in part the offspring of the primary mesoblast," but this cannot 

 be said of Amphitrite. 



We have records of the origin of the mesoderm in the follow- 

 ing forms : among annelids, Clepsine, Nereis Dummerillii (free- 

 swimming and suppressed trochophore). Nereis limbata, Spio, 

 Aricia, Polymnia, Amphitrite ; among molluscs, 7Vm/zW, Plan- 

 orbis. Umbrella, Unio, and Crepidula; among polyclades, Dis- 

 cocoelis. A comparison of the mesoblast formation in these 

 forms yields additional evidence for the homology of cleavage 

 cells. In all except Clepsine, Nereis Dnmmerillii, and Disco- 

 ccelis, the original mesoderm cell arises from the same one of the 

 fourth generation of cells, which has descended from D of the 

 4-cell stage. The impaired ^nesodenn cell belongs to the ideal 

 6^-cell stage, and is one of the second quartette of cells counting 

 from, the vegetative pole, and always arises by a left-oblique 



