Fig. 8.--A transverse section through the dorsal region of a forty-five hours' embryo; _ao_, aorta with a few blood-corpuscles. v, Blood-vessels, all of them being formed in the splanchnopleure, and all of them provided with the secondary investment of Klein; _pe_, pellucid area; _op_, opaque area.
Fig. 9.--Small portion of a section through the opaque area of a thirty-five hours' embryo, showing protoplasmic processes, with nuclei passing from the somatopleure to the splanchnopleure.
Fig. 10.--Section through the heart of a thirty-four hours' embryo. _a_. Alimentary canal; _hb_, hind brain; _nc_, notochord; _e_, epiblast; _so_, mesoblast of the somatopleure; _sp_, mesoblast of the splanchnopleure; _hy_, hypoblast; _hz_, cavity of the heart.
Fig. 11.--Section through the same embryo as fig. 10, and passing through the orifice of the omphalomeseraic vein. _of_, Omphalomeseraic vein; other references as above.
These two sections shew that the heart is entirely formed from the mesoblast of the _splanchnopleure_, and that it is formed by the splitting of that part of the mesoblast which has turned to assume its normal direction after being folded in to form the muscular wall of the alimentary canal. In fig. 11 the cavities so formed on each side have not yet united, but in fig. 10 they have united. When the folding becomes more complete the cavities (_of_, _of_) in fig. 11 will unite, and in this way the origin of the omphalomeseraic veins will be carried further backwards. In the section immediately behind section 11 the mesoblast had become thickened, but had not split.
V. A PRELIMINARY ACCOUNT OF THE DEVELOPMENT OF THE ELASMOBRANCH FISHES[10].
Footnote 10: From the _Quarterly Journal of Microscopical Science_, Vol. XIV. 1874. Read in Section D, at the Meeting of the British Association at Belfast.
During the spring of the present year I was studying at the Zoological Station, founded by Dr Dohrn at Naples, and entirely through its agency was supplied with several hundred eggs of various species of Dog-fish (Selachii)--a far larger number than any naturalist has previously had an opportunity of studying. The majority of the eggs belonged to an oviparous species of _Mustelus_, but in addition to these I had a considerable number of eggs of two or three species of _Scyllium_, and some of the Torpedo. Moreover, since my return to England, Professor Huxley has most liberally given me several embryos of _Scyllium stellare_ in a more advanced condition than I ever had at Naples, which have enabled me to fill up some lacunæ in my observations.