NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 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 haematoxylin has been used 
chiefly, while Heidenhain’s iron hematoxylin and Grenacher’s alcoholic 
borax carmine have both given excellent results. 
b. METHOD OF STUDY. 
Tn 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. Now 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 hematoxylin 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 different 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. 
lt is evident from a comparison of Locy’s photographic representations, 
given in his final paper ('95), with his drawings, that the latter are, 
