No. 3.] , THE CELL-LINEAGE OF NEREIS. 437 



mesoblasts " from which the mesoblast-bands arise. The neuro- 

 nephroblast divides into two, four, and finally eight cells (telo- 

 blasts), from which the outer strata of the germ-bands arise. 

 The eight teloblasts arrange themselves in two groups of four 

 each ; each of these gives rise to the germ-band of the corre- 

 sponding side, which is composed of four corresponding rows of 

 cells. Of these rows the lowermost {i.e. the one that ultimately 

 lies nearest the median ventral line) gives rise to the corre- 

 sponding half of the ventral nerve-cord, and is, therefore, called 

 the neural row, the corresponding teloblast being accordingly a 

 neuroblast. The adjoining two rows were believed by Whitman 

 to be concerned in the origin of the nephridia, and were hence 

 termed nephric rows, their teloblasts nephroblasts. The fourth, 

 whose history is doubtful, is the outer row, produced by the 

 outer or lateral teloblasts. Inside these four rows is the 

 mesoblast-band, the whole structure constituting the "germ- 

 band." The structure and origin of the germ-band, as thus 

 described, has been fully confirmed in the case of Clepsme, and 

 I have found an almost precisely similar structure in the germ- 

 bands of L^mibidctis (No. 29). Bergh (No. i) has confirmed 

 both Whitman's account and my own, but asserts that the 

 "nephroblasts" and lateral teloblasts give rise to the circular 

 muscles, and hence are to be regarded as myoblasts. 



In Limibricus, unfortunately, the origin of the neuro-nephro- 

 blasts in the cleavage has not yet been determined. In Rhyji- 

 chelmis, however, Vejdovsky describes the origin of a group of 

 cells that, as Whitman points out, are almost certainly to be 

 identified with them, although Vejdovsky did not determine their 

 later history. Of these cells there are at first two ("first and 

 second mesomeres ") successively budded forth from the large 

 macromere. This is followed by the separation from the same 

 macromere of a "third mesomere," which divides longitudinally 

 to form the two primary mesoblasts. The remainder of the 

 large macromere enters into the formation of the mesenteron, 

 as in Nereis. The first and second mesomeres divide so as to 

 form four large cells lying in the ectoblast at the posterior ex- 

 tremity of the incipient germ-bands, and there is scarcely room 

 to doubt that these four cells represent the four posterior telo- 

 blasts of Nereis and the eight (at one time four) of Clepsine and 

 Lumbricus. 



