R 
Normal Plates of the Development of Squalus acanthias. 57 
is elongated and much expanded dorsally. The boundary between the thickened lateral walls and the epen- 
dymal roof is indicated by a dotted line. Between the rhombo-mesencephalic fissure and the anterior end 
of this dotted line is a thickened segment of the roof of the hind-brain, the anlage of the cerebellum (Cb.). 
The spinal chord needs no particular description. The area of origin of the olfactory nerve (N.olf.) is indicated 
on the wall of the fore-brain. The oculo-motor nerve (N.oec.) is similar in position and relations to that of 
the preceding reconstruction. The trigeminal nerve, however, exhibits several important changes. Above the 
mesocephalic ganglion (G.mes.) arises two minute branches, the upper of which is the superficial ophthalmic ramus 
(R.oph.sup.V). The mandibular and maxillary branches of the trigeminus extend ventrally to the base of 
their respective arches and are shown in the reconstruction as cut off near their distalends. The ganglionic 
mass of the facial-acoustic complex is divided definitely into a posterior acoustic part (N.ac.) and an anterior 
facial portion. The latter shows some subdivision into hyoid, buccal, and superficial ophthalmic portions. 
From the anterior end of the last the superficial ophthalmic ramus (R.oph.sup.VII) extends forwards 
to a point anterior to the cephalic edge of the ganglion of the trigeminal nerve. The hyoid ramus 
(R.hyo.V11.) extends to the base of the hyoid arch. The glossopharyngeal nerve (N.g.-ph.) needs no special 
description. Four branchial divisions of the vagus ganglion (N.vag.) are present in this embryo. They are 
represented as cut off near their origin from the general ganglionic mass. The visceral ramus (R.vic.X) is 
prolonged backward dorsally and laterally to the oesophagus to the mid cardiac region. A slight 
enlargement of the posterior dorsal angle of the general ganglion of the vagus represents the point of 
origin of the lateral line ramus. The upper dorsal ganglionic commissure (Comm.d.) connects the posterior 
border of the vagus ganglion with the first and second spinal-occipital ganglia. These two structures 
which appear only as enlargements of the dorsal ganglion of the commissure are not connected with 
their ventral roots which are seen directly below them. There are fourty seven spinal ganglia (Sp.g.) in 
the trunk region anterior to the posterior end of the cloaca. The dorsal ganglionic commissure, which 
until this stage has connected the spinal ganglia, is now broken down between the anterior twelve ganglia 
and between five ganglia of the mid trunk region. The entire length of the spinal nerves has been 
represented in the reconstruction. Their elongation in the regions of the pectoral and pelvic fins is quite 
noticeable. Sympathetic ganglia (Symp.) are present upon the ventral rami of the spinal-occipital nerves 
and anterior forty-one spinal nerves. The dorsal rami of the spinal nerves are represented as cut off near 
their origin from the nerve trunk. 
The optic vesicle (Op.v.) is elongated and deeply cup-shaped. In it is lodged a large egg-shaped 
lens (Ls.). The otocyst (Oto.) needs no special description. The ductus endolymphaticus (D.endol.) still opens 
to the ectoderm by a long furrow. 
It has been necessary to give a rather diagrammatic view of the pharynx because of the great growth 
of the gill clefts. These structures are represented as cut off close to their origin from the pharynx. The 
hypophysis (Hyp.) extends forward from the anterior end of the pharynx and lies in contact with the upper 
wall of the infundibulum. It is slightly expanded at its cranial extremity. The thyreoid gland (T%.) is 
somewhat obscured in the figure by the ventral aorta. Its dorsal outline, however, is represented by dotted 
lines. It can be seen that it is attached to the pharynx along the anterior half of its dorsal surface. The 
pharynx is abruptly constricted immediately behind the fifth gill cleft, passing over into the rather elongated 
oesophagus. The stomach, which is obscured in the figure by the lateral lobes of the liver (L.), is 
but little larger in caliber than the oesophagus. That portion of the gut which represents the duodenum 
(Duo.) is slightly less in caliber than is the stomach, and is directed a little ventrally; this bending is the 
first indication of the duodenal loop. The vitelline duct (D.vit.) now enters mainly on the right side of the 
intestine. Eight turns of the spiral valve ($Sp.v.) are present. The intestine is much constricted just anterior 
Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. XII, 8 
