No. I.] GERM-LAYERS IN CLEPSINE. 157 



is well-marked. The contrast between the nephridial {iipli) 

 and the lateral {in') cells is equally strong, so that in this ad- 

 vanced stage of development it is still easy to find all the 

 derivatives of the neuro-nephric stratum, and to connect them 

 with the primary cell-rows shown in Fig. 6. 



3. The Larval Gland-Cells. 



Passing now to the anterior end of the same embryo we find 

 the nerve-cord presenting the same general form, with the 

 median and lateral parts more clearly defined (Fig. 23). The 

 median part does not yet show the double commissures. In 

 this section, taken in the region marked gl in Fig. 8, we meet 

 with a very interesting larval organ, consisting of numerous 

 large gland-cells, each with its own duct leading to the exterior. 

 These massive gland-cells lie between the sub-cesophageal 

 ganglia and the epidermis, and extend over an area of greater 

 breadth than the ganglia. These glands arise as a pair of thick- 

 enings of the epidermal layer immediately behind the cephalic 

 lobe, and appear as rounded prominences in quite an early 

 stage of the germ-bands. (Fig. i). The epidermal thickening 

 is always clearly distinct from the neural cells, as may be seen 

 to best advantage in longitudinal sections (Figs. 20 and 21). It 

 is undoubtedly this thickening which Nusbaum has figured and 

 described as the basis of the nerve-chain. Only the deeper 

 cells of the thickening are destined to become gland-cells, 

 and these appear to sink gradually beneath the surface, the 

 ducts forming in situ rather than by subsequent outgrowths. 

 A median sagittal section of this stage (Fig. 28) often shows 

 only a few gland-cells compared to the number met with in 

 sections passing about midway between the median ventral line 

 and the side of the embryo (Fig. 29), showing that they still 

 form two more or less distinct groups. 



Nusbaum (No. 8, p. 28) has described a " provisional dorsal 

 organ " entirely different from anything I have seen. " Chez 

 I'embryon, dont le systeme nerveux est deja completement 

 separe de I'ectoderme, j'ai remarque, au milieu de la parol 

 dorsale du corps, dans la troisieme partie anterieure de sa 

 longueur, une couche de cellules hautes, cylindriques, de I'ecto- 

 derme. . . . Les cellules ectodermiques, qui forment cette 

 proeminence, emettent ensuite des fils externes, minces, tres 



