442 MR. C. H. O DONOGHUE ON THE 
any definite tributaries corresponding to the nutrient branchial 
veins and the posterior facial vein described by Parker in 
Mustelus. 
3. 11. The Posterior Cerebral Vein * (Parker) ['Text-fig. 2, P.C.] 
commences at the anterior end of the cerebellum and passes back- 
wards along its side. ‘Towards the posterior end of the cerebellum, 
after receiving a well-marked tributary coming from its mid-dorsal 
region, it runs out laterally over the sac-like lateral dilatation 
of the posterior choroid plexus. Here it drains a remarkable 
venous network, a kind of rete mirabile, composed of quite 
well-marked veins. Its path now lies along the lateral border of 
the median portion of the posterior choroid plexus, from which 
it receives numerous tributaries, and just before the calamus 
seriptorius it passes out through the cranium closely apposed to 
the dorsal surface of the tenth cranial nerve. It parts company 
with this nerve outside the cranium and apparently passes 
through the muscles to the front end of the anterior cardinal 
sinus, but it is difficult to follow. The fine veins from the 
membranous labyrinth appear to open into the posterior cerebral 
vein at its inner end. Parker describes and figures the veins 
not as passing through the cranial wall but as running backwards 
to unite and form the myelonal vein. 
Grosser has figured this vein in an embryo of Sceyllium 26 mm, 
long (2 a, fig. 4), but does not call attention to it in the text. In 
dealing with Zriton and Salamandrina, however, he mentions 
this vein as leaving the skull with the vagus nerve, and points out 
that it is of general occurrence in many of the higher groups, a 
fact also noted by Gaupp. It was readily made out in the sections 
of embryos of 37 and 56 mm. length that were examined. 
3. 11. The Myelonai Veins (Parker) [Text-fig. 2, M.]. Two 
well-marked mvelonal veins are present, a dorsal and a ventral. 
The dorsal myelonal vein runs the length of the spinal cord, 
and at the anterior end forks just behind the calamus scriptorius. 
The two limbs of the fork run into the posterior cerebral vein 
just as it is leaving the cranial cavity. Parker describes this 
vessel as forming a rhomboidal plexus in each vertebral segment, 
but although the anterior end of the vein may be more or less 
double for a short way behind the point where it divides, there 
is no sign of this arrangement in Scylliwm. Segmental veins 
are given off, and these often tend to form a lateral vessel on 
each side of the cord by anastomosing in a longitudinal direction ; 
it appears to be very irregular, however. 
The ventral myelonal vein is formed by the union of two 
* This vein and the anterior cerebral are very difficult to inject, as it is almost 
impossible to insert even a hypodermic needle into them. However, I found that they 
could be made distinct in the following way. After the anterior cardinal vessels 
of a freshly killed fish have been quite filled with injection mass and plugged, the 
cranium is dissected away so as to expose as much of the brain as possible. A 
fixing fluid, corrosive formol, is slowly injected into the dorsal aorta until the 
cranial arteries begin to get colourless and the veins distended with blood. If the 
anterior end of the fish in this condition is placed in 5 per cent. formalin overnight, 
it will be found that the blood in the veins has coagulated and become dark in colour. 
