4 1 2 Wilson: [ vol . vi . 



of the median line and showing two of the entoblast nuclei) is 

 shown in Fig. ZZ, XX. There is no segmentation cavity, and 

 the entomeres are everywhere closely surrounded by the one- 

 layered ectoblast. The section shows the exact relations of the 

 primary mesoblast {M) to the cells of the ventral plate i^o-p). 

 Just anterior to the primary mesoblast is a crowded group of 

 cells, some of which lie below the surface and soine of which 

 contain bhnsJi-black pigment-gTaritdes. This group of cells are 

 certainly in part the offspring of the primary mesoblast and 

 form the beginning of the pigment-area {p.a.) which has so 

 great a significance in the later history of the larva. These 

 cells I shall call the secondary mesoblast. Close examination 

 shows a distinct line of demarcation between the secondary 

 mesoblast cells and those of the ventral plate, but this boundary 

 is not so distinct as it afterwards becomes. 



The primary mesoblasts continue to bud forth secondary 

 mesoblast cells for some time after their enclosure. At length, 

 however, the budding ceases and the mesoblasts divide longi- 

 tudinally into two somewhat unequal parts (Figs. 6i, 6^^, of 

 which the two mesial cells are the larger. By subsequent divis- 

 ions of the same character (Figs. 77-82) each primary mesoblast 

 gives rise to a gro7ip of cells that extends npwards on each side 

 along the cleavage-line between the anterior and posterior pairs 

 of entomeres, i.e. along the line of the first cleavage and at right 

 angles to the prototroch. These two groups of cells are the foun- 

 dations of the mesoblast-bands, zvhicJi have, therefore, from the 

 beginning a position at right angles to the margins of the blasto- 

 pore ; i.e. at right angles to their direction in Lumbricus, Cleph 

 sine, and Rhynchelmis. 



Meanwhile the pigment-area increases in size and becomes 

 very distinctly marked off from the surrounding cells (Figs. 

 79-82, XIX). The pigment is at first arranged in two somewhat 

 V-shaped masses (Fig. 79), but in later stages these become 

 more or less completely connected together and surrounded by 

 scattered granules that occupy a fairly-well defined circular area 

 at the lower pole of the trochophore. The paratroch is subse- 

 quently developed around the margijis of this area (Figs. 85, 90), 

 and still later the anus appears in the centre of the paratrochal 

 ring. Hence the pigment-area represents the posterior extremity 

 of the adult body. 



