1% Normal Plates of the Development of Squalus acanthias. 5I 
The fore-brain is still connected with the skin ectoderm in the region (Ant.neur.) where the anterior 
neuropore was formerly present, but this fusion is much less extensive than that seen in the reconstruction 
of the embryo of 4.8 mm. The thalamic nerve crest (O.thal.) is still connected with the trigeminal crest (C.trig.) 
by a slender cellular strand. The dorsal portion of the trigeminal crest or urtrochlearis (O.urtr.) is con- 
nected with the general mass of the trigeminal crest by an extremely irregular cellular band. That portion 
of the trigeminal crest which lies in the mandibular arch becomes indistinguishable from the mesenchyma 
ventrally, and its ventral termination is therefore only indicated by a broken line. The facial-acoustic crest 
(C.fac., C.ac.) is somewhat enlarged at the level of the dorsal aorta. Here it is possible to distinguish between 
facial and acoustic portions, and the latter has been represented in coarse stippling. A small branch of 
the facial-acoustic crest passes a short distance backward over the external surface of the otocyst. The 
facial-acoustic crest, like the trigeminal crest, becomes indistinguishable from the mesenchyma ventrally. The 
Comm.d. 
Curtr. O:trig. Cfac. (ac. _Oto. 0.g-ph. O.vag. G.vag.T. Ao. | Sm.r. G@.sp.0c. 
Be I (U En & ii {N N N 1 MM ERR 
V.vit.s. 
Op.w. 
S.ceph. Or.pl. Ao.a.I. Gl.c.I. 4Ao.a.II. Th. D.mes. 
Ant.neur. V.cap.med. Gl.c.II. Gl.p.ILI. 
Fig. 9. 
otocyst (0fo.) is deeply invaginated, but is still broadly connected with the skin ectoderm. The glosso- 
pharyngeal crest (N.g.-ph.) is completely separated from the vagus. The vagus crest (N.vag.) extends ventrally 
to the upper edge of the pharynx. From its ventral border there is a small projection (@.va.I) which 
probably represents the ganglion of the first branchial ramus of the vagus. There is also a small posterior 
prolongation, from which the remaining branchial ganglia probably arise. The vagus crest is broadly 
connected with the dorsal ganglion commissure (Comm.d.). The first spinal-occipital ganglion (G.sp.-0c.) 18 
represented by an enlargement of this structure just above the posterior end of the heart. 
The pharynx ends bluntly in front, and gradually narrows to form the oesophagus (Oes.) posteriorly. 
The first two gill slits (Gl.s.) are open in part, and the third gill pouch (@l.p.) is in the process of formation. 
The thyreoid gland (7%.) appears as a small pouch projecting backward from the floor of the pharynx 
behind the second gill slit. The oesophagus is decidedly flattened transversely. At the level shown in cross 
section in the figure, it is about to join the vitelline duct. Its ventral part at this point is expanded into 
an ill defined pocket, the anlage of the liver (Z.). 
The mesothelial walls of the heart are represented in stipple. The dorsal mesocardium is in part 
broken down, and the dorsal connection thus established between the sides of the pericardial cavity is 
zu 
