No. I.] GERM-LAYERS IN" CLEPSTNE. 



155 



germ-bands ; and I confess that I do not see wherein this view 

 requires ' any special support.' At the time Balfour penned 

 the above criticism he evidently was not aware that my observa- 

 tions on the origin of the nervous system in Clepsine were but 

 little more than a corroboration of those of an eminent Russian 

 embryologist." 



{b) Observations on the Origin of the Nerve-chain. — The proof 

 that the entire ventral nerve-chain arises as two simple lon- 

 gitudinal rows of cells, and that each row is produced by the con- 

 tinued proliferation of a single cell, — the neuroblast, — is to be 

 obtained by the study of surface-preparations in connection with 

 sections made in the planes of the three axes. Sections show 

 that the fourfold striated appearance of the germ-bands is due 

 to the presence of four rows of cells beneath the epidermal stra- 

 tum of each band ; and surface-preparations enable us to trace 

 these rows forward into the special organs developed from them, 

 I have been helped towards precise results by the differential ac- 

 tion of the preservative fluids employed, and by natural distinc- 

 tions between the neural and the nephric cell-rows. These 

 distinctions are less conspicuous in the species hitherto studied 

 in Europe than in the American species (C. parasita), and 

 hence have been overlooked. The nephridial rows are more 

 granular and stain more deeply with osmic acid than the neural 

 and lateral rows. A glance at Plates IV. and V. will show how 

 extremely useful such distinctions have been in the analysis of 

 the neuro-nephric stratum. 



Having already briefly indicated the different layers of the 

 germ-bands, it remains to consider more in detail the elements 

 composing the neuro-nephric stratum. As was made clear in 

 my former paper (No. i), there are exactly four rows of cells 

 in this stratum in each germ-band. The outlines of these rows 

 can easily be seen in germ-bands hardened in situ (Fig. i, PI. 

 IV.), but better in surface-preparations which have been freed 

 from the yolk (Fig. 8, PI. V.). They are most distinctly 

 marked at the hind ends of the bands, but can be traced for- 

 ward nearly to the cephalic lobe in many preparations. The 

 reason for their becoming less and less sharply marked as we 

 pass from the hind end forwards, lies in the fact that develop- 

 ment is more and more advanced in this direction. Behind, the 

 rows are simple («>., each consists of a line of single cells), and 



