Normal Plates of the Development of Squalus acanthias. 61 
of the two anterior tubules of the pronephros (Proneph.). The anterior mesonephric tubule is not connected 
with the body cavity. The three following tubules are connected with the body cavity, and overlap the 
Wolffian duct laterally but are not connected with this structure. The remaining tubules (Mes.t.) shown in 
the figure are connected with both the coelom and Wolffian duct and possess two marked sacculations. 
They may be compared in their stage of development with those of an embryo of Pristiurus 17 mm. in length 
reconstructed by RAaBL and illustrated in Fig. 17 of Plate XVII, of his “Theorie des Mesoderms’”. 
The pericardial cavity is now distinctly cut off from the coelom along the median line by the 
septum transversum (S.trans.). The heart needs no particular description. As stated above the branchial 
vessels are represented as cut off near their origin. The deep ophthalmic artery (A.oph.) is seen extending 
forward lateral to the hypophysis and dividing into two branches above the optic stalk. The vitelline 
artery (A.vit.) arises from the dorsal aorta by one trunk at the level of the septum transversum. The large 
veins passing into the sinus veinosus are in a similar stage of development to that described and figured 
for the embryo I5s mm. in length, except that the lateral vein (V.lat.) now enters the sinus venosus 
between the common cardinal and vitelline vein. 
Fig. 16. Graphic reconstruction of an embryo 20.6 mm. in length. Normal plate series No. 28. 
(EIBER@ET7OA) ESEL: 
This is the last of the series of reconstructions of the general anatomy presented in this paper. 
The general method of representation of the various structures is similar to that employed in Fig. ı2. The 
pharynx, however, is indicated only in outline, because any representation of the great expanded gill clefts 
would obscure other structures. The optic vesicle has been omitted for the same reason. 
The reconstruction shows the great enlargement of the telencephalon vesicles (Telen.). The floor 
of the fore-brain is also directed somewhat upward anteriorly. The diencephalon is somewhat constricted 
at its cephalic end, and its roof is now almost entirely occupied by the well marked paraphysis (Pa.) and 
epiphysis (Epi.). The latter structure is decidedly dilated at its distal extremity. Although not distinctly 
indicated in the figure there is a slight subdivision of the diencephalon into parencephalon and syncephalon. 
The infundibulum (Inf.) is markediy enlarged and the well marked mammillary process is directed 
anteriorly. The mesencephalon (Mesen.) is markedly dilated and the rhombo-mesencephalic fissure (F\r.-mes.) 
now indents the lateral wall of the brain from dorsal to ventral surface. The anlage of the cerebellum 
is represented by a decided thickening of the roof of the hind-brain. The oculomotor nerve (N.oc.) extends 
backwards as far as the notochord. The trochlearis (N.tr.) is represented by a small filament arising from 
the dorsal surface of the brain in line with the rhombo-mesencephalic fissure. The trigeminal nerve (N. trig.) 
is somewhat obscured by the overgrowth of the ganglion of the facial nerve. Its ganglion is “A”-shaped in 
outline, and the mesocephalic ganglion (G.mes.) is decidedly enlarged. From the ganglionic bridge connecting 
these two structures there extends forward as small filament, the superficial ophthalmic ramus of the trigeminus 
(R.oph.sup.V). The deep ophthalmic ramus (R.oph.pro.V) extends downward a short distance beyond the 
mesocephalic ganglion. The ganglion of the facialis shows distinct division into three parts; the ganglia of 
the ramus ophthalmicus superficialis (@.r.oph.sup. VII), the ramus buccalis (G@.r.buce. VII) and the ramus hyoideus 
(G.r.hyo.VII). From the ventral surface of the last of these springs a small filament, the palatin ramus 
of the facial nerve. The acoustic ganglion covers a small portion of the ventral and lateral surfaces 
of the otocyst. The glossopharyngeal ganglion (G.g-ph.) is represented as cut off near its origin from 
its dorsal surface and a small nerve filament, the supra-temporal ramus (R.s-#. IX) extends upward along 
the external surface of the posterior canal pocket. The four branchial rami of the vagus (@.br.X) are 
represented as cut off near their origin. The lateral line ramus (R.lat.X) of the vagus extends backwards 
