438 WILSON. [Vol. VI. 



Now there can be no question that the ectoblastic portion of 

 the germ-bands in Liimbriciis and RJiyncJichnis is the homo- 

 logue, on the one hand, of the corresponding portion in the 

 Hirudinea, and on the other hand, of that structure in the 

 Polychaeta that I have called the ventral plate. In the Hiru- 

 dinea and Oligochaeta this structure is from the beginning 

 separated into two halves, which subsequently grow together 

 along the median ventral line. In the Polychaeta the two halves 

 are always distinguishable, but sometimes (as in Lopadoi'hyncJms 

 or in PolygordiiLS) they develop in their definitive position side 

 by side ; sometimes (as in Nereis) they are primarily in contact, 

 but secondarily separate and again come together along the 

 median ventral line, as in the Oligochasta. 



It follows that the first soviatoblast of Nereis is the homologiLe 

 of the primary nenro-nephroblast of Clepsine, with which it pre- 

 cisely agrees both in mode of origin and in ultimate fate. It 

 differs only in the fact that it gives rise on each side to two 

 instead of four teloblasts. In Rhynchelmis it is represented by 

 the first and second " mesomeres " taken together, and it would 

 be interesting to determine whether these two are already dif- 

 ferentiated from one another {e.g. one a neuroblast, the other a 

 nephroblast or myoblast). In Nereis I have been unable to dis- 

 tinguish between the products of the inner and outer pairs of 

 the four teloblasts; this may, however, be owing simply to the 

 fact that the cell-rows become indistinguishably fused together. 



It is clear without further discussion that the second somato- 

 blast of Nereis is the hoinologue of the ^^ third meso^nere'' of 

 Rhynchelmis and of the primmy viesoblast of Clepsine before its 

 division into two. 



Further than this I shall not carry the comparison. It has, 

 I think, been shown that representatives of the three leading 

 groups of annelids show an extraordinarily precise correspond- 

 ence in the cleavage of the ovum, the origin of the germ-layers, 

 and the differentiation of the principal elements of the germ- 

 bands ; and this correspondence warrants the confident ex- 

 pectation that the apparently aberrant mode of development 

 represented by LopadorhyncJuis will be found to conform to the 

 ordinary type when a detailed study of the early stages shall 

 have been made. 



