Normal Plates of the Development of Squalus acanthias. 59 
sinus venosus receives on either side a 
Symp. 
R.mes 
Spv 
Panc. 
‚Mes.t. 
vitelline vein (V.vit.) and a common cardinal 
(V.card.com.). The latter breaks up into the VB -L > 
posterior and anterior cardinals in the usual 
Duo. 
manner, 
Fig. 15. Graphic reconstruction from 
W.d. 
sagittal sections of the anterior 
D.vit. 
half of an embryo 18.0 mm. in 
length. Normal plate series No. 27. 
(H.E.C. 204.) X 33: 
The plan used in the representation 
VW 
& 
D.chol. 
@.bl. 
of this reconstruction is the same as that 
Sıtrans. 
employed in Figs. ıı and 12. It has been 
> 
x 
necessary, however, to omit the optic vesicle 
| 
V.lat. V.vit. 
and lens, and to represent the gill slits as 
—— 
cut off close to their origin from the pharynx. 
The use of sagittal sections does not permit 
X X V.card.com. St. Proneph. Avit. 
| 
of accurate reconstruction of the branchial 
R.lat. R.vie. 
B.cor. 
blood vessels so these structures are also 
G.sp.0C. 
represented as cut away close to their origin. 
The widely expanded telencephalon 
\ 
(Telen.) now extends forward decidedly over 
NSS 
Th 
Fig. 15. 
| the roof of the tube-like diencephalon. The 
\ 
optic stalk (Op.s.) is shown in cross section. 
X N.vag. Comm.d. 
Its anterior wall is markedly thickened by 
the invasion of optic fibers. The epiphysis 
\ 
G.epib. G.epib. 
D.endol. IX 
(Epi.) and paraphysis (Pa.) are seen as two 
prominent structures springing from the roof 
R.ımd.V  R.hyo. VII 
of the diencephalon. The infundibular sack 
v 
/ 
S 
is somewhat obscured by the structures 
v1 
overlying it. It is bounded above by 
shallow lateral grooves. The mesencephalon 
is somewhat expanded and is sharply 
separated from the hind-brain by the deep 
rhombo-mesencephalic fissure (F\r.-mes.). The 
anlage of the cerebellum forms a small 
G.epib. 
N.trig. N.fac. VII N.ac. N.ab. Oto. 
but distinct thickening on the roof of the 
hind brain. There is no distinct line division 
between the myelencephalon and the cord. 
The oculomotor nerve (N.oec.) extends back- 
wards to the level of the optic stalk. The 
R.oph.sup.V No. 
mesocephalic ganglion (G@.mes.) is connected 
with the anterior end of the main ganglion 
