latly 
nites 
mid- 
Irsal 
very 
iites. 
axial 
sur- 
tinct 
\ome 
yo- 
° six 
y to 
‚same 
third 
Iree- 
s in 
Last 
ıted. 
t di- 
roxi- 
two- 
mid- 
juite 
ıture 
3 eX- 
dian 
s of 
. No 
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. tips 
obli- 
little 
En 
FE 
anni 
nn 
Normal Plates of the Development of Squalus acanthias. 
35 
nn 
Head-Somites 
Notochord, Subnotochordal Rod, Axial Skeleton 
Ectoderm in 
early Stages 
| 
No. 
Much like preceding stage 
(No. 26). 
Anterior somite appears as 
a slightly thickened mass of 
mesenchyma between pre- 
mandibular somite and optic 
vesice. Median stalk of 
premandibular somite very 
slender, perforated by a 
minute lumen. Lumen of 
hyoid somite obliterated; 
external and dorsal surfaces 
mesothelial, median and ven- 
tral surfaces fibrous. Middle 
third of hyoid somite (anlage 
of external rectus) extends 
ventrally and laterally to 
dorsal wall of optic vesicle. 
Anterior somite entirely ob- 
literated. Cavity of pre- 
mandibular somite, except 
for lateral-ventral angle, fil- 
led with loose mesenchyma. 
Outline of premandibular 
somite barely distinguish- 
able. Superior oblique di- 
stinctly cross striated. Ad- 
ductor mandibulae faintly 
crossstriated. Inferiorrectus, 
inferior oblique, and muscle 
“F” fibrous. 
Outlines of lateral portions 
of premandibular somite en- 
tirely obliterated. Median 
portion present as a small 
fibrous cord lying between 
infundibulum and recurved 
part of notochord. Cavity 
of mandibular somite ob- 
literated. Inferior rectus, in- 
ternal rectus, inferior ob- 
lique and adductor mandi- 
bulae muscles distinctly 
cross striated. 
Anterior end of notochord recurved for 50 u. (by sections), ending 40 u. anterior to hypophysis. The tip is separated 
from the median stalk of premandibular somite by a small cushion of cells containing many vacuoles. Mesenchyma 
slightly thickened about notochordal sheath in trunk region. — Subnotochordal rod one-eighth diameter of notochord 
in trunk region. — Parachordals represented by a very slight condensation of mesenchyma on either side of noto- | 
chord, extending from anterior end of otocyst to glossopharyngeal nerve. This thickening extends laterally to 
mesial edges of ganglia. 
Notochord recurved anteriorly for I00 u, recurved portion very small. Division of sheath into elastica interna and 
externa. Mesenchyma much condensed about notochord, condensations on dorso-lateral angles to form neural 
arches. — Subnotochordal rod extends anteriorly to anterior limit of mouth, separated from notochord by little 
mesenchyma in pharyngeal region. One-fifteenth diameter of notochord in trunk region. — Parachordals extend 
forward to anterior edge of root of trigeminal nerve, posteriorly become indistinguishable from anlagen of neural | 
arches. Mesenchymous in anterior fourth, precartilaginous posteriorly. Expanded laterally at otocyst. Beginnings | 
of mesial and lateral processes about base of otocyst. First anlagen of alisphenoids as slight condensations of, 
mesenchyma, extending from hypophysis to basal zone of hind-brain. Mandible, hyoid, and first branchial arch 
indicated by masses of slightly thickened mesenchyma. 
Notochord nearly 0.5 mm. in diameter in anterior trunk region. Sheath as in preceding stage (No. 28). Neural arches 
in trunk region precartilaginous, extending dorsally to the middle of the spinal cord. Haemal arches represented 
by thin condensations of mesenchyma on either side of ventral half of notochord. Ribs represented by masses of 
thickened mesenchyma which extend laterally to mesial edge of somites. — Subnotochordal rod extends from second 
gill slit backward beyond cloaca. About one-twentieth dıameter of notochord in trunk region. Surrounded by a 
definite non-cellular sheath. — Parachordals slightly expanded where joining alisphenoids. Posterior to exit of 
trigeminal nerve, appear in cross section as thin plates lying on either side of notochord, continuous with the 
thickened mesenchyma surrounding it. About as broad and one-sixth as thick as notochord. In region of otocyst, 
parachordals wedge-shaped in cross section, thicker portion lying laterally. Processes from parachordals extend 
dorsally on internal and external walls of otocyst, forming base of periotic capsule. Remainder of periotic capsule 
represented by thickened mesenchyma. Alisphenoids extend forward on either side of brain as thickened mes- 
enchymal processes, as far as anterior edge of optic 1% Bases of alisphenoid plates precartilaginous posteriorly, 
end ventrally abruptly at level of oculomotor nerve. Posterior to this point they gradually slope inward, joining 
parachordals between exits of trochlear and trigeminal nerves. A thickened plate of mesenchyma, lying just below 
telencephalon, represents the rostrum. Posterior to the optic nerve, this plate is divided into two parts (the trabe- 
culae), which extend upward on either side of infundibulum, and end ın a precartilaginous thickening at dorsal 
surface of infundibulum. Slight thickenings beneath each optic cup represent ethmoid processes. Mandible, 
including processus palatino-basalis outlined in densely thickened mesenchyma, extends forward beyond posterior 
end of infundibulum. Hyoid and all branchial arches outlined in densely thickened mesenchyma. 
Notochord in trunk region approximately 0.5 mm. in diameter. Sheath 5 u thick. Neural arches distinct con- 
densations of mesenchyma, triangular in cross section, extending dorsally to middle of cord. Beginning of trans- 
verse processes as flattened thickenings of mesenchyma on ventral half of each side of notochord. Ribs ill defined 
thickenings of mesenchyma confluent with transverse processes proximally and extending distally to mesial edge 
of somites. — Parachordals much the same as in preceding stage (No. 29), extending somewhat farther laterally. 
Outer wall of otic capsule extends almost to dorsal surface of otocyst. Outer and inner walls attached to para- 
chordals along middle third of capsule, and fused before and behind, enclosing otocyst. Alisphenoids much as in 
preceding embryo (No. 29) except that a larger portion of plates has become precartilaginous. Ventral processes 
of thickened mesenchyma extend downward on either side of diencephalon, half way to optic nerve. Rostral process 
faintly outlined in thickened mesenchyma; extends forward to anterior edge of nasal pit. Posterior to rostrum, 
trabeculae pass upward on either side of infundibulum, ending in slight enlargements just below parachordals. 
Posterior portion of trabeculae precartilaginous. Position of mandibular, hyoid, and first branchial arches indicated 
by condensations of mesenchyma. 
A definite layer of sheath cells between inner 
arches much as in preceding stage (No. 30). 
and outer elastic sheaths of notochord dorsally and ventrally. Neural 
Transverse processes extend farther laterally. Rib anlagen attached 
to transverse processes by constricted mesenchymal thickenings. In mid-trunk region, ribs extend distally to inner 
edge of lateral ramus of vagus. — Subnotochordal rod, in trunk region, approximately one thirty-fifth diameter of 
notochord. — Alisphenoids extend upward over ventral two-thirds of mesencephalon. Precartilaginous ventrally, 
mesenchymous dorsally. Mesenchymous ventral processes from alisphenoids extend downward to level of optic 
nerve. Rostrum widely expanded, composed of thickened mesenchyma; extends forward to nasal pits. Ethmoid 
process represented by faint condensation of mesenchyma which extends as far forward as nasal pits; fused posteriorly 
with rostrum below eye. Constriction between rostrum and trabeculae proper very marked in region of optic 
chiasma, the connecting plate hardiy wider than the neural tube above it. Trabeculae separate a short distance 
behind optic chiasma. Posterior ends of trabeculae extend upward on either side of hypophysis almost meeting para- 
chordals, but not fused with them. Trabeculae precartilaginous posterior to optic chiasma. Mandibular, hyoid, and 
first three branchial arches outlined in thickened mesenchyma. 
Neural arches and transverse processes 
In trunk region, two or three layers of elongated notochordal sheath cells. 
trunk region. Intercalary plates in- 
precartilaginous. A thin layer of precartilage surrounds notochord in anterior 
dicated by thickenings of mesenchyma? kibs precartilaginous proximally: extend outward to lateral edge of 
musculature. — Subnotochordal rod barely distinguishable. — Alisphenoids meet over mid- and hind-brain. Their 
dorsal two-thirds thickened mesenchyma, remainder precartilage. Supraorbital ridge outlined in thickened mes- 
enchyma. Rostrum extends forward to anterior edge of optic cup, forming a broad flat plate anteriorly and con- 
stricted to a narrow bar posteriorly. Trabeculae, except for posterior tips, fused along median line. Posterior ends 
fused with parachordals, thus enclosing caudal end of hypophysis. Visceral arches precartilaginous. Processus 
palatino-basalis of mandibular arch separated from trabeculae only by a little mesenchyma. 
Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. XII. 5 
27 
29 
30 
& 
DS 
