No. 2.] THE MESO BLAST-BANDS IN ANNELIDS. 213 



the ventral ectoblast, as Salensky describes it. From their 

 point of origin the bands, each now represented by a group 

 of six or seven cells, extend upwards between the outer layer 

 and the four entoblast-cells, along the line of contact of the 

 two anterior cells (C, D) and the two posterior {A, B). Their 

 later history will be described elsewhere. 



Let us now return to the progeny of X. Increasing rapidly 

 in number, both by their own divisions and by the addition of 

 cells formed from the four posterior teloblasts, they give rise 

 to a broad, bilobed plate, consisting throughout of a single layer 

 of granular cells, and occupying the greater part of the lower 

 half of the embryo. The prototroch is developed from a series 

 of micromeres, at first single, that encircles the equatorial belt 

 of the embryo, and lies immediately behind the four posterior 

 teloblasts. The latter persist for a considerable period, but 

 ultimately disappear. The two outer ones first break up into 

 smaller cells, and as this takes place, the remaining two separate 

 from each other along the median line. Thus the ventral plate 

 becomes bilobed behind, with a V-shaped area between the two 

 lobes, and a single teloblast at the tip of each. This teloblast 

 remains until each half of the ventral plate contains fifty or 

 more cells, still lying quite at the surface. Ultimately it dis- 

 appears, and the proctodseum is formed in the anterior part 

 of the V-shaped area. At a still later period the ventral plate 

 thickens, becoming several layers deep on each side the median 

 line, and gives rise to the neural plates and the seta-sacs. Its 

 relation to the nephridia and the circular muscles is still under 

 investigation. 



Let us now compare these facts with the development of 

 Clepshie and LopadorhyncJms. In Clepsine the large posterior 

 macromere first separates off a single micromere (as in Nereis), 

 and then divides into two large cells. The upper right-hand 

 cell (" neuro-nephroblast " of Whitman) has precisely the same 

 relation to the rest of the embryo as the first pro-teloblast, X, 

 of Nereis. In Clepsine this cell breaks up into eight teloblasts, 

 viz. (still using Whitman's terminology) : two neuroblasts, four 

 nephroblasts, and two lateral teloblasts ; and these give rise to 

 all the structures of the ventral body-wall excepting those aris- 

 ing from the mesoblast-bands ; i.e. the neural foundations, a part 

 of the nephridia, some of the ventral ectoblast (probably), and 



