230 MEAD. [Vol. XIII. 



latter and used plugs of absorbent cotton covered with thin 

 linen paper to prevent losing the minute eggs in the cotton. 

 The vials were then packed in glass jars with labels toward 

 the outside, and the whole jar filled with strong alcohol. 



In staining for surface views I have always obtained the best 

 results with Delafield's haematoxylin, especially with Conklin's 

 method of using the stain diluted and acidulated. The stained 

 eggs are cleared in clove oil or cedar-wood oil in the ordinary 

 way, and mounted in balsam, the cover glass being supported 

 by four paper "feet." A little clove oil allowed to mix with 

 the balsam keeps the latter soft, so that for weeks the &^g can 

 be rotated and drawn with the camera, in any position. 



In drawing surface views with the camera lucida, when the 

 cell outlines are faint and complicated, I have used thin black 

 paper and a soft lead pencil, the point of which has been painted 

 with Chinese white. The advantages of this method are twofold ; 

 the image of the whitened pencil-point and that of the egg are 

 both more distinct, and the pencil lines on the paper are never 

 confused with the lines of the image. By simply smearing the 

 back of the paper with soft lead and stub, the sketch may be 

 easily transferred to drawing paper. 



In the belief that it is unwise to depart from a nomenclature 

 which involves nearly all the observations in this particular type 

 of cleavage, unless a greatly supewor one can be substituted, 

 I have adopted that introduced by Wilson in his Nereis paper,^ 

 and employed with certain modifications by Heymons^ and 

 Lillie^ (Kofoid's* ingenious system of naming the cells avoids 

 possible objections to Wilson's system, but is fraught with 

 certain practical disadvantages). I have, however, amended 

 Wilson's scheme in the following points to adapt it to com- 

 parative description.* 



1 Wilson, E. B.: The Cell Lineage of Nereis. Journ. of Morph., vol. "VI, 

 1892. 



2 Heymons: Zur Entwicklungsgeschichte von Umbrella mediterranea. Zeitschr. 

 f. wiss. Zool. Bd. 56, 1893. 



3 Lillie, F. R. : The Embryology of the Unionidae, A Study in Cell Lineage. 

 Journ. of Morph., vol. X, no. i, 1895. 



* Kofoid: On Some Laws of Cleavage in Limax. Proc. Ainer. Acad. Arts and 

 Set., January, 1894. 



* Some of the points have been mentioned by Kofoid and Lillie. 



