214 WILSON. [Vol. IV. 



perhaps to other structures.^ In Nereis the corresponding cell 

 breaks up into four instead of eight teloblasts, which give rise 

 likewise to those parts of the ventral body-wall not derived from 

 the mesoblast-bands ; i.e. the neural plates, the ventral ecto- 

 blast, the seta-sacs, and probably other structures (for the pres- 

 ent I leave the nephridia and the circular muscles out of consid- 

 eration). The foregoing considerations render it practically cer- 

 tain that the first p7'o-teloblast of Nereis {X) is the homologue of 

 the " neuro-nephroblast" of Clepsine. In Clepsine the second and 

 larger cell produced by the division of the large posterior ma- 

 cromere (which I shall call the common primary mesoblast) 

 divides into equal parts, which become the primary mesoblasts, 

 and give rise to the mesoblast-bands, in the usual manner. The 

 second pro-teloblast (V) of Nereis is therefore the homologue of the 

 common primary fnesoblast of Clepsine. It therefore appears 

 that Clepsine and Nereis agree in every essential feature of de- 

 velopment. They differ only in secondary details, — in the ulti- 

 mate number and arrangement of the teloblasts, and in the fact 

 that they and their products are at first superficial and form a 

 part of the outer layer, — falsely called the ectoblast. It is in- 

 teresting to recall in this connection the fact that in Litmbricns 

 eight of the ten teloblasts likewise remain for a long time at the 

 surface. 



Turning now to Lopadorhynchus, we find here also a bilobed 

 ventral plate, as in Nereis, but no teloblasts, in the earliest stages 

 thus far observed. From it arise, as in Nereis, the mesoblast- 

 bands, the neural plates, and the seta-sacs ; and I think the ven- 

 tral plate must be regarded as completely homologous in the 

 two forms. They differ only in the earlier segregation and dif- 

 ferentiation of the mesoblastic material in Nereis which leads to 

 the formation of a pair of transitory teloblasts, which, however, 

 form part of the ventral plate. Meyer makes the interesting 

 statement in a recent paper ^ that he has satisfied himself that 

 the neural elements and the mesoblast-bands do not arise in 

 Lopadorhynchus from a common foundation, but from " Zwei 

 verschiedene, nur dicht zusammengedrangte Bildungsheerde." 



^ Bergh asserts that the lateral teloblasts and the nephroblasts give rise, in Lum- 

 bricus, to circular muscles, and are therefore myoblasts. I shall return to this question 

 in a later paper. 



"^ Biol. Centralblatt, i Juli, 1890. 



