230 Transactions. — Zoology. 



fore-brain, where there arc no thickenings for optic tlialarni, corpora striata, or 

 lobi inferiores, and no constriction of the prosocoele to form a Y-shaped 

 " foramen of Monro " like the third and lateral ventricles. 



The second (ii.)> third (hi.), 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 circular 

 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 I,"* 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 (e), in the small number 

 of myotomes, and in the great size of the somatic or umbilical stalk (so.s) by 

 which the embryo is attached to the yolk-sac. 



* Conip. Embryol., vol. ii., p. 378. 



* " The Development of Elasmobranch Fishes " (Journ. of Anat. and Phys., vol. x., pi. 

 xxiv.) 



