Normal Plates of the Development of Squalus acanthias. 55 
marked infundibulum with which the hypophysis (Hyp.) is in contact. The epiphysis (Epi.) and paraphysis 
(Pa.) form two distinct swellings from the roof of the diencephalon. The sulcus rhombo-mesencephalicus 
is well marked. The optic vesicle (O.pv.) is now decidedly cup-shaped, and the lens (Zs.) is completely 
4 separated from the ectoderm. The otic vesicle (Oto.) is connected with the skin ectoderm by a long 
slender endolymphatic duct (D.endol.). The thalamice and urtrochlear crests have entirely disappeared. The 
first of the eye muscle nerves, the oculomotor (N.oc.) is well established and is connected at its distal end 
with the ciliary ganglion (@.cil.). The ganglion of the trigeminus (N.irig.) is shaped like an inverted V, as 
in the preceding stages. There is still present a narrow ganglionic filament — the remains of the old 
urtrochlear process (Pr.urt.). The mesocephalic ganglion (G@.mes.) is expanded. It is connected by a few 
slender strands with the ciliary ganglion (@.cil.), which lies directly mesial to it. Both the mandibular (R.md.V) 
and the maxillary (R.mx.V) ramus of the trigeminus are well developed. The facial-acoustic nerve complex 
(N-fa.-ac.) is also well advanced. The hyoid ramus (R.hyo.VII) extends half way down the second arch. 
' The superficial ophthalmic ramus is absent on the left (reconstructed) side but is present on the right side. 
N.g.-ph. Comm.d. 
No. Nitrig. N.fa.ac. Oto. D.endol. | Nvag. G.r.lat.X | G.sp.oc. 7 Sp. Avit. Proneph. Mes.t. 
P.urt. 
@.cil. 
2 Q [|| ARE @l.s. Ao.vent. Rovis.X 8. At. L.a. G.bl.L.l. Dit. Panc. L.g. Spw. R.mes. 
| Epi. V 
= 
Pa. Is. Opw. *Hyp. A.oph. R.mx.V Fig. 12. 
The glossopharyngeal (N.g.-ph.) and vagus (N.va.) crests are connected by a narrow dorsal commissure. The 
three anterior branchial ganglia of the vagus are established, and that part of the crest lying posterior to 
these is slightly divided into an upper lateral ramus portion (G.r.lat.X) and a lower portion, from which the 
\ remaining branchial ganglia will form and from which the visceral ramus (R.vic.X) will arise at a later stage. 
No special description need be given of the spino-occipital and spinal nerves. The dorsal and ventral roots 
of the latter have united, although in the reconstruction they are indicated as cut away proximal to the 
point of union. | 
The pharynx is markedly elongated and the first five gill slits open to it. The hypophysis 
(Hyp.) is well marked. The thyreoid now lies between the two ventral aortae and is completly 
hidden by them. The main divisions of the liver are present at this stage. A small anterior diverti- 
culum (L.a.) extends forward between the vitelline veins.. The left lateral pouch (L.l.) extends upward 
a little above the dorsal wall of the gut, and is joined below with the median chamber. From the base of 
the median chamber projects a small rounded pocket, the anlage of the gall bladder (G.bl.). The pancreas 
(Panc.) is still broadly attached to the dorsal wall of the gut but the connecting stalk is somewhat con- 
stricted transversely. The groove (L.g.) in the left wall of the vitelline duct (D.vit.) is a little more pro- 
