66 KARM NARAYAN BAHL 



nings of five or six coelomic cavities in each of the two bands. 

 On examining, however, the surface epiblast covering these 

 mesoblastic bands ventrally, we can distinguish four rather 

 large and rounded cells on each side, called the teloblasts. 

 These teloblasts lie a little way in front of the pole-cells : the 

 three ventral ones lie six or seven cells in front of the pole- 

 cells as seen in longitudinal sections (fig. 2), while the fourth, 

 the lateral teloblast, lies a little farther forward than the rest. 

 In these very young embryos (figs. 1-4) the teloblasts form part 

 of the surface epiblast, but can be easily distinguished from 

 the adjacent epiblastic cells both by their larger size and by the 

 fact that their nuclei are free from granules surrounding 

 the nucleolus and thus give an appearance of greater trans- 

 parency as compared with the nuclei of the other cells. Of 

 these four teloblasts on each side the one near the mid-ventral 

 line is the neuroblast, going to form the nerve-cord of the 

 adult, the two lying outside the neuroblast are the nephro- 

 blasts, which go to form the nephridia, wiiile the outermost 

 and dorsal is the lateral teloblast which hes just outside 

 and dorsal to the coelomic sac on each side at this stage of 

 development of the embryo (fig. 4). In a series of transverse 

 sections of an embryo, about 300 m in length, from which 

 figs. 3, 4, and 6 are taken, we can follow these four teloblasts 

 forwards as they bud off rows of cells in front. The rows of 

 cells in front of the teloblasts can be followed for a long way 

 in young embryos. Concerning the nephridial cells in con- 

 tinuation with the nephro blasts, we have to note that 

 while the nephroblasts are large cells and occupy the whole 

 thickness of the epiblast, the nephridial cells in front are small 

 and come to lie deep in the ectoderm. They can be seen 

 distinctly marked off by a sort of boundary line from the 

 definite epiblast, which is very thin at places where these 

 nephridial cells occur. These cells are thus embedded in the 

 ectoderm, as shown in figs. 3, 4, and 6, but they have not yet 

 formed a separate layer of their own. 



The next step in the development of the nephridia, which 

 is slower than that of the nerve-cord, is that the nephridial 



