230 Transactions.—Zoology. 
fore- ion, here th thiekeni tic thalami, corpora striata, or 
lobi inferiores, and no constriction of the Jonan to orta a Y-shaped 
“ foramen of Monro " like the third and lateral ventricles. 
The second (ii), third (iii.), and fourth (iv.) nerves have the usual 
relations; springing from the anterior end of the medulla oblongata are 
three chief roots (v., vii., viii.), which I had not the opportunity of tracing, 
but which, judging from analogy, must be the roots of the fifth, seventh, 
and eighth nerves: of these one is dorsal in position and posterior to the 
others, and is evidently the root considered by Balfour* as the ramus 
dorsalis of the seventh, which goes largely to form the ramus ophthalmicus 
superficialis of the orbitonasal nerve. A small backwardly-directed nerve 
behind these roots is probably the glossopharyngeal (ix.); and several large 
roots towards the posterior end of the medulla the vagus (x.) 
There is nothing of special interest about the nasal sacs, and in the eyes 
the only points I have to mention are the extreme dilation of the cireular 
pupils, and the presence of a beautiful argentea interna or silvery tapetum in 
contact with the whole extent of the retina. According to Owen this silvery 
layer of the choroid is internal also in Galeus. The auditory organ has the 
usual structure. 
6. Embryology. 
The few observations I have to make under this head are concerned 
almost entirely with the external characters of the three stages found in the 
uteri of the specimen dissected. 
First stage.—Of the ten foetuses, one was very considerably younger than 
the rest, and had a length of about 7 mm. It is represented in fig. 11. In 
general form it corresponds pretty nearly with Balfour's * Stage L,"* but 
presents many differences of greater or less importance. 
The head is very sharply separated from the trunk, which latter is 
«strongly arched dorsally and much compressed from side to side. The tail 
(c) is quite short but quite clearly differentiated from the trunk and sharply 
bent round against the left side. From this latter circumstance it would 
seem that active movements had already begun, as in Balfour’s “I,” but 
the embryos were all dead when I received them. There is as yet no trace 
of a caudal fin, but the tail can hardly be said to be dilated terminally. 
Other resemblances to *I" are found in the fact that the cerebral flexure 
is far from complete, the fore-brain (f.b) being still in advance of the mid- 
brain (m.b), in the imperfect condition of the eye (c), in the small number 
of myotomes, and in the great size of the somatic or umbilical stalk (so.8) by 
which the embryo is attached to the yolk-sac. 
* Comp. Embryol., vol. ii., p. 378. 
* ' The Development of Elasmobranch Fishes” (Journ. of Anat. and Phys., vol. x., pl. 
xxiv.) 
