NEAL: NEKVOUS system IX SQUALUS ACAXTHIAS. 155 



the precipitation of osmium in the very earliest stages of development, 

 and it has given me results which I have been able to obtain in no other 

 ■way. For staining sections Kleinenberg's hsematoxylin has been used 

 chiefly, while Heidenhain's iron hsematoxylin and Greuacher's alcoholic 

 borax carmine have both given excellent results. 



h. Method of Study. 



In studying Locy's " neural segments " in Squalus embryos, reflected 

 light was used, and in consequence low powers of the microscope were neces- 

 sary. I have used constantly a small Zeiss stand in which the upper half 

 of the stage and the superstructure revolve on the lower half of the stage, 

 and his objective A and ocular I. My method of procedure has been, 

 first, to make with the aid of a camera lucida an outline of the embryo 

 cleared in clove oil and viewed as a transparent object. The irregularities 

 of the edge of the neural plate may thus be represented accurately, 

 and may serve as landmarks in the subsequent study of the specimen as 

 an opaque object. After the outline drawing has been made, the speci- 

 men is transferred to a watch glass filled with alcohol. Xow the im- 

 portant question is illumination. In order to bring out the delicate 

 structures along the edges of the neural plate, oblique illumination should 

 be used, since it brings into strong contrast the shadows and the high 

 lights. The embryos should be rotated, so that light may be obtained 

 successively from all directions and thus the chance of deception by 

 false lights avoided. As the embryo is studied chiefly from the ventral 

 side (for reasons given by Locy), careful manipulation with brush and 

 needle is necessary in order to remove the yolk, which would otherwise 

 obscure the edges of the neural folds. In studying these surface con- 

 ditions, I have found a very faint heematoxyliu stain and a black back- 

 ground to be of advantage. 



In representing the specimen under observation, I have not had 

 recourse to photography, but have made as faithful a representation 

 as possible with pencil, seeking to preserve the relative values of light 

 and shade. Since it is possible by careful illumination to increase the 

 contrast of light and shade to a considerable amount, it is well to study 

 the same embryo with difiereut kinds of illumination. In this way it is 

 possible to determine more satisfactorily what is permanent and what is 

 not. The study of the segments is by no means easy, and the labor 

 is considerable because it is necessary to study so many individuals. 

 It is evident from a comparison of Locy's photographic representations, 

 given in his final paper ('9.5), with his drawings, that the latter are, 



