ms mo 
A sau 
Normal Plates of the Development of Squalus acanthias. 
25 
Head -Somites 
Notochord, Subnotochordal Rod, Axial Skeleton 
Ectoderm in 
early Stages 
No. 
Lumen of premandibular somite circular in cross section, extends back- 
ward to tip of pharynx. Median connecting stalk broad anteriorly, 
tapering posteriorly. Dorsal wall epithelial, remainder mesenchymous. 
Two small cavities in stalk on either side of median line. A distinct 
lumen in anterior somite dorsally. 
. 
Distinct lumina in premandibular somite. Anterior somite connected 
with premandibular on either side by a strand of thickened mesenchyma. 
Pre on of mesenchyma from mesial wall of hyoid somite. Hyoid 
somite separated from fourth head somite by a deep lateral groove. 
Anterior somite extends nearly to cranial end of mandibular somite; its 
mesial wall fused with lateral wall of premandibular somite. Premandi- 
bular somite triangular in cross section, the two lateral cavities connected 
by a broad dorsal and a minute ventral stalk. From the dorso-median 
stalk two small blind pouches extend posteriorly. Posterior end of hyoid 
somite divided into dorsal and ventral processes. 
Anterior somite almost circular in cross section; walls epithelial and 
thick, with two rows of nuclei ventrally and mesially. Extends slightly 
in front of premandibular cavity. Posterior wall fused with premandi- 
bular, but no communication of cavities. Premandibular somite much 
larger than in preceding stages; lateral portions connected by a single 
median stalk, the lumen of wich is extremely small at the median line. 
Three irregular chambers bud off dorsally from this stalk. A small process 
with distinctly epithelial walls extends from the dorso-lateral angle of 
the mandibular somite, between the root of the semilunar ganglion and 
the hyoid somite. Ventral mesial angle of hyoid head somite decidedly 
mesenchymous. 
Lateral parts of premandibular somite connected by broad median stalk 
containing a large lumen, which is prolonged backward as an irregular 
pocket. Walls of anterior, premandibular and mandibular somites in con- 
tact above eye, but not fused. 
Anterior somite elongated, narrow, lying anteriorly in agroove in lateral wall 
of premandibular somite and posteriorly in the angle between premandibular 
and mandibular somites. On the left side, lumen of anterior head somite opens 
into that of the premandibular somite by a channel 50 u in diameter. Median 
stalk of premandibular somite much compressed between infundibulum and 
notochord (see note under latter). A large pocket extends upward from the 
mesial surface of the mandibular cavity, and is separated from it anteriorly 
by the root of the trigeminal nerve. “Muscle E” *) represented by a thicken- 
ing at the end of this pocket? Hyoid cavity distinctly bi-lobed posteriorly. 
No communication between lumina of anterior somites and of premandi- 
bular somite. Processes of premandibular somite which go to form 
inferior oblique muscle and inferior rectus muscle distinct. Anterior 
process (anlage of superior oblique muscle) of mandibular somite extends 
a far. cranially as anterior wall of optic vesicle, “Muscle E” process 
ıstinct. 
*) The nomenclature of A. LAMmB (Amer. Journ. Anat., I, 1901, 185), 
has been followed in descriptions of rudimentary eye muscles. 
XII 
Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. 
Anterior tip of notochord slightly recurved. Central cluster of nuclei, 
as described for preceding stages (Nos. I7 and 18), in head and pos- 
terior trunk region. In anterior trunk region, nuclei more generally 
scattered and smaller in size. From level of cloaca backward, the 
primitive cellular condition of notochord still present. — A separate 
anterior segment of subnotochordal rod springs from roof of pharynx 
near its anterior tip and extends forward just under notochord to its 
ventral curve. Second segment a ridge on dorsal surface of pharynx 
from level of first gill slit to middle heart region. Extends backward 
to cloaca as a free rod lying between two dorsal aortae. Fused with 
entoderm at cloaca. 
Anterior end of notochord bent ventrally at an angle of 90°. In 
contact but not attached to median stalk of premandibular somite. 
Nuclei of notochord generally scattered from level of facial nerve 
backward to posterior trunk region. In other parts of notochord, 
cells arranged in central cluster. — No anterior segment of subnoto- 
chordal rod. 
Anterior end of notochord a little recurved, in contact but not fused 
with median stalk of premandibular cavity. Notochord 120 u in 
diameter in largest part. Central cluster of nuclei at extreme anterior 
end, and from level of anterior end of cloaca backward, nuclei in 
trunk region gathered at the periphery of notochord. — Subnotochordal 
rod arises as in preceding embryos (Nos. 20 and 21), fuses with ento- 
derm again just posterior to cloaca. In posterior part of free portion 
is increased in size until one-half diameter of notochord. 
Anterior end of notochord recurved for 24 u. (by sections). In cardiac 
and hepatic regions, cells arranged in a fairly definite epitheloid layer 
at periphery or notochord. — Subnotochordal rod attached anteriorly 
to roof .of pharynx between third and fourth gill pouches, posteriorly 
to postanal gut just behind cloaca. Much reduced in size in pharyngeal 
region. 
Anterior end of notochord recurved for 50 u. (by sections). Recurved 
part in contact with median stalk of premandibular somite. Notochord 
150 u. in diameter in largest part (region of yolk stalk). Cells generally 
scattered in head region backward to third gill slit. From this level 
backward to anterior end of cloaca, cells flattened against elastica. 
Cells gathered in central cluster or central ring from posterior end of 
cloaca to end of notochord. — Subnotochordal rod entirely detached 
from gut. In contact but not fused with entoderm over third gill slit. 
Extends backward behind cloaca. 
Anterior end of notochord recurved for 110 y. (by sections). Broadly | 
oval in cross section. Nuclei throughout notochord flattened against, 
elastica. — Subnotochordal rod extends from first gill slit nearly to | 
posterior end of notochord. At level of second gill slit, approaches | 
roof of pharynx but is not fused with it. A distinct median ridge on| 
pharynx at this point. Average diameter of subnotochordal rod 15 ». 
Anterior end of notochord sharply recurved for 150 u. Broadliy oval 
in cross section in trunk region; vertical diameter I8o u, transverse 
diameter Iso u. Elastica much thickened. — Subnotochordal rod in 
contact with roof of pharynx over third gill slit. 
20 
21 
22 
23 
25 
26 
