Normal Plates of the Development of Squalus acanthias. 49 
(Or.pl.). There are two gill pouches (@.p.I, II) — the first is a little more prominent than the second. The 
mid-gut is still broadly connected with the blastodermic entoderm; this connection, however, is much con- 
stricted transversely. The thickened plate of cells found at the anterior end of this connection, and so 
prominent a feature of younger embryos, is still present though in a less marked degree, in this specimen. 
The cloaca (Cl.) is represented by a very slight dilatation of the hind-gut — its floor is in contact, but not 
fused, with the skin ectoderm. The hind-gut is very slightly constricted behind the cloaca, but is broadly 
connected with the neurenteric canal (Neur.e.). 
The anterior end of the notochord (No.) is bent sharply ventrally and merges into the preoral 
mesoderm. Its posterior end fuses with the entoderm and ectoderm of the anterior wall of the neurenteric 
canal. The hypochorda or subnotochordal rod (Sn.r.) forms a prominent median ridge on the roof of the 
gut, extending from the level of the second gill slit nearly to the end of the mid-gut. It is connected 
with the gut by a narrow neck along the median line. 
Fig. 8. Graphic reconstruction of an embryo 4.8 mm. in length. Normal plate series No. 18. 
(ERE.@ 771398.) 933: 
The plan of this figure is similar to that of Fig. 7 of an embryo 4.0 mm. long, with the following 
exceptions, The skin ectoderm is represented as cut along the median line in the neuropore region, and 
as left in position over the oral plate and the first gill slit. The vascular system and a part of the peri- 
cardium are included in the reconstruction. The arteries are shaded with light broken transverse lines. 
The veins are represented in black crossed with white lines. The heart and pericardium are shaded in 
heavy stipple. The mesothelial layer of the heart is cut away at either end of the tube. The pericardium 
is represented as cut away dorsally above the dorsal mesocardium, and ventrally along the median line. 
Such a section exposes the internal surface of the right side of the pericardium. 
Curir. Otrig. @l.c. O.fa.ac. C.g-ph.  O.vag. 
\ 
\ \ Ao.dors. Sr. 
O.thal. 
Ant.meur. V.cap.med. | |Or.pl. Th. Ao.went.| H. Vvit.s. yo 
S.ceph. 4o.a. D.mes. x 
Fig. 8. 
The fore-brain vesicle is much dilated. The anterior neuropore is now completely closed, but the 
skin and neurectoderm are still fused in this region (Ani.neur.). The optic vesicle (Op.v.) is similar to that 
seen in the reconstruction of the embryo 4.0 mm. in length. The trigeminal nerve crest (C.trig.) is broadly 
attached to the hind-brain and the posterior part of the mid-brain. The anterior part of this attachment 
extends upward to the dorsal median line. This portion of the crest represents the urtrochlearis (O.urtr.) 
of Dozrn and other authors. The connection between this anterior part and the main part of the crest is 
interrupted by two small openings which indicate the line along which the separation between the urtrochlearis 
and the trigeminus proper will take place. The ventral part of the trigeminal crest is divided into an 
anterior and a posterior limb. The anterior or ophthalmie portion lies mainly between the optic vesicle 
and the brain tube. It is continuous anteriorly with the thalamic crest (C.thal.). The posterior limb or 
Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. XII, fi 
