44 Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. 
venous system as a whole is only represented in Figure Io, a reconstruction of a embryo 7.5 mm. in length. 
Four reconstructions of the head somites follow the reconstructions of the general anatomy. The oldest 
embryo thus reconstructed is 9.0 mm. in length. For the later history of these structures in Squalus 
acanthias the reader is referred to the excellent paper by A. Lam!) which includes a series of recon- 
structions of older stages. 
Most of the reconstructions have been made from transverse sections and in the majority of cases 
every other section was drawn and used in the preparation of the figure. A few reconstructions have been . 
made from sagittal sections by KASTSCHENKo’s?) method but when this has been done it is noted in con- 
nection with the description of the figure. It has been necessary to use several scales of magnification in _ 
the series as the smallest embryo is less than one-tenth the length of the largest one. 
All the figures of reconstructions with the exception of the simple cut of the aortic arches (Fig. 14) 
are the work of Mr. Wm. OLıver. A liberal grant from the National Academy of Sciences made it possible 
to secure the services of this accomplished draughtsman. The use of various schemes of shading to 
represent different systems and organs is explained in the descriptions of figures. The dissections are 
all drawn with the camera lucida at a magnification of six diameters. It is hoped that they will bring 
out certain features, such as the general proportions of the viscera at different stages of development, 
which are not well illustrated by the reconstruction method. 
Fig. 1. Graphic reconstruction of an embryo of 1.8 mm. in length. Normal plate series No. 6. 
(H.E.C: 984.) X so. 
The embryo is seen from the left. Uncut surfaces of ectoderm are shaded in longitudinal lines; 
entoderm is shaded in vertical lines; mesoderm is shaded in stipple. The yolk upon which the embryo rests 
is represented by diagonal black lines crossed with white. Ectoderm in cross section is indicated by oblique 
lines running from left to right; entoderm in cross section, by oblique lines running from right to left; 
mesoderm in cross section, in broken lines running from right to left. In the head region the ectoderm 
has been cut away at the median line. Behind the cephalic plate the ectoderm is cut to the left of the median 
line. The external surface of the archenteron 
RER, and the lateral plate of mesoderm are ex- 
posed in the anterior part of the embryo. 
In the posterior part of the embryo the ecto- 
derm, mesoderm, and entoderm are all cut 
Fig. 1. 
back to a plane a little to the left of the 
median line. The entoderm in the head and anterior part of the trunk region is cut along the line where 
it spreads out over the blastodisc as a flat sheet. 
The archenteron (A.) is seen to be folded up into a shallow pouch, from the anterior end of which 
the preoral gut (P.g.) projects forward a little into the slight head fold. Laterally, the walls of the arch- 
enteron flatten out abruptly. The mesoderm (M.) appears as a narrow plate, triangular in cross section, Iying 
above the archenteron. The anterior end of this plate is still fused with the entoderm and is indicated 
with a broken line. 
D)#Eoeicit. 
2) N. KASTSCHENKO, 1886, Methode zur genauen Rekonstruktion kleinerer makroskopischer Gegenstände. Arch. f. Anat. 
u. Phys., p. 388. 
