No. 3-] THE CELL-LINEAGE OF NEREIS. 373 



thus ensured. Most of the eggs I have studied were deposited 

 about 9 P.M., but in a few cases they were not laid until an hour 

 or two later. The cleavage was repeatedly followed through 

 the entire night on the living ova, and most of the cleavage- 

 stages have been observed scores of times. Owing to the 

 warmth of the lamp, the eggs develop more rapidly under the 

 microscope than in the aquarium, so that, by keeping the aqua- 

 rium cool and taking fresh material after the completion of each 

 division, every step in the cleavage may be observed twice or 

 more in the same lot of eggs. 



I have found it best to examine the eggs on an ordinary glass 

 slide under a very long narrow cover-glass, one end of which is 

 supported by wax feet. The eggs are drawn up in a pipette 

 and run under the cover-glass from the upper end, whereupon 

 they arrange themselves in a single layer. By moving the cover- 

 glass the eggs may be rolled, though with some difficulty owing 

 to the presence of oil-drops which cause the eggs to lie with 

 the animal pole downwards. In later stages, after the larvae 

 begin to swim, they may be paralyzed by adding to the water a 

 few drops of a weak solution of cocaine in dilute methyl alcohol. 

 The cilia may thus be brought to a standstill and their arrange- 

 ment easily studied. The embryos may also be colored ijitra 

 vitam to any desired extent by adding a one per cent aqueous 

 solution of methyl-blue to the water. This method does not 

 give a differential staining, but is very useful in certain stages 

 by rendering the protoplasm and cell-outlines more distinctly 

 visible. 



For preserving the embryos various methods were employed. 

 For sections the best hardening fluids are Flemming's fluid 

 (Fol's weaker formula), Perenyi's fluid, sublimate, and chrom- 

 acetic acid, especially the two former. Kleinenberg's picric 

 acid, which gives beautiful results with many annelid larv^, 

 and which was successfully employed by v. Wistinghausen for 

 N. Diimerilii, I have found unsatisfactory. It is, furthermore, a 

 curious and instructive fact that Lang's sublimate acetic mixture, 

 which V. Wistinghausen found useless, works very well with 

 the American species. These reagents were employed in the 

 usual manner, the eggs being left in them from ten to thirty 

 minutes. 



These methods are, however, of small value in comparison 



