“I 
wo 
PROFESSOR A. MILNES MARSHALL. 
On the Heap Cavitirs avd Assocratep Nerves of Etasmo- 
BRANCHS. By A. Mitnes Marsnatt, D.Sc., M.A., Fellow 
of St. John’s College, Cambridge. Professor of Zoology in 
Owens College. (With Plates V and VI). 
Tue discovery by Mr. Balfour! of the extension forwards to 
the head of that splitting of the mesoblast which in the trunk 
gives rise to the body cavity, and of the subsequent division 
of the cavity so formed into the series of segments which he 
has termed head-cavities, has given us a new and very impor- 
tant clue to that favourite problem of morphologists, the seg- 
mentation of the vertebrate head. 
I have been led to pay special attention to the development 
of these head-cavities in Elasmobranchs, in order to test the 
accuracy of conclusions as to the morphology of certain of the 
cranial nerves, notably the third pair, to which I had been led 
by a study of their development in the chick.? 
In the present paper I propose to treat of—(1) the develop- 
ment of the head-cavities, (2) the relations of the cranial nerves 
to these cavities, (3) certain stages in the development of those 
nerves which are most intimately connected with the cavities, 
and (4:) certain stages in the development of the eye muscles. 
For my material, which consists almost entirely of embryos of 
Scyllium canicula, | am indebted partly to Mr. Balfour and 
partly to the Managers of the Southport Aquarium. With few 
exceptions, the embryos were hardened in a + per cent. solution 
of chromic acid, to which a few drops of a 1 per cent. solution 
of osmic acid were added. In this solution they were left for 
twenty-four hours, and then transferred to alcohol of 30 per 
cent., which was gradually increased in strength until absolute. 
In embryos prepared in the above manner, the brittleness due 
to the use of osmic acid alone is completely avoided; all the 
epiblastic tissues are stained a deep brown or black colour, 
and the nerves in particular stand out with remarkable sharp- 
ness and distinctness from the surrounding and but slightly 
stained mesoblast. 
Much of any success I may have obtained is due to this 
mode of preparation, which appears to be peculiarly applicable 
to nerve investigation. For a knowledge of the method I am 
again indebted to Mr. Balfour. 
The Development of the Head-Cavities.—As Balfour has 
* *Elasmobranch Fishes,’ p. 206, seq. 
* Vide this Journal, January, 1878, p. 23, seg. 
