Beer, B., On the development of the Sylvian fissure in the human embryo. Journal of Anatomy and Physiology, 1890.

Béraneck, E., Recherches sur le développement des nerfs crâniens chez les lézards. Recueil zoolog. Suisse, 1884 ; Étude sur les replis médullaires du poulet. Recueil zoolog. Suisse, iv., 1887. Chiarugi, Sullo sviluppo di alcuni nervi cerebrali e spinali. Anat. Anzeiger, 1889.

Cunningham, D. J., The complete fissures of the human cerebrum, and their significance in connection with the growth of the hemisphere and the appearance of the occipital lobe. Journal of Anatomy, April, 1890.

Dohrn, A., Die Entstehung der Hypophysis bei Petromyzon Planeri. Mittheil. aus der zoolog. Station zu Neapel, iv., i., 1883; Veber die erste Anlage und Entwicklung der motorischen Rückenmarksnerven bei den Selachiern, Mittheil. aus der zoolog. Station zu Neapel, Bd. viii., 1888.

Ewart, J. C., On the development of the ciliary or motor oculi ganglion. Proc. Roy. Soc.. xlvii., 1890.

Froriep, A., Ueber ein Ganglion des Hypoglossus u. Wirbelanlagen in der Occipitalregion, Arch. f. Anat. u. Physiol., Anat. Abth., 1882; Ueber Anlagen von Sinnesorganen am Facialis, Glossopharyngeus und Vagus und über die genetische Stellung des Vagus zum Hypoglossus und über die Herkunft der Zungenmusculatur, Arch. f. Anat. u. Physiolog., Anat. Abthl., 1885.

Goette, A., Ueber die Entstehung und die Homologien des Hirnanhangs, Zool. Anzeiger, 1883. Golowine, E., Sur le développement du système ganglionnaire chez le poulet, Anat. Anzeiger, 1890 Graaf, H. W. de, Bijdrage tot de kennis van den bouw en de ontwikkeling der epiphyse bij Amphibien en Reptilien. Proefschrift, Leiden, 1886.

His, W., Ueber das Auftreten der weissen Substanz und der Wurzelfasern am Rückenmark menschlicher Embryonen, Archiv f. Anat. u. Phys., Anat. Abth., 1883; Zur Geschichte des menschlichen Rückenmarks und der Nervenwurzeln, Abhandl. d. math.-phys. Kl. d. kgl. Sächs. Gesellsch. d. Wissensch., Bd. xiii., No. 6, 1886; Die Entwicklung der ersten Nervenbahnen beim menschlichen Embryo. Uebersichtliche Darstellung, Archiv f. Anat. u. Physiol., Anat. Abth., 1888; Zur Geschichte des Gehirns sowie der ventralen und peripherischen Nervenbahnen beim menschlichen Embryo, Abhandl. d. math.phys. Kl. d. kgl. Sächs. Gesellsch. der Wissensch., Bd. xiv., 1888; Die Formentwicklung des menschlichen Vorderhirns, Abhandl. d. königl. Sächsischen Gesellschaft, Bd. xv., 1889; Die Neuroblasten u. deren Entstehung im embryonalen Mark, Abhandl. d. königl. Sächs. Gesellschaft, Bd. xv., 1889. His, W., jun., Zur Entwicklungsgesch. d. Acustico-facialis-gebictes beim Menschen, Arch. f. Anat. u. Physiol., Anat. Abth. 1889.

Hoffmann, C. K., Ueber die Metamerie des Nachhirns u. Hinterhirns u. ihre Beziehung z. d. segmentalen Kopfnerven bei Reptilienembryonen, Zool. Anzeiger, xii.

Johnson, A., and Sheldon, Lilian, On the development of the cranial nerves of the newt, Proceed. of the Royal Society, vol. xl., 1887.

Kaczander, J., Ueber die Beziehungen des Medullarrohrs zu dem Primitivstreifen, Wiener Medicin. Jahrb., 1886.

Kraushaar, R., Entwicklung der Hypophysis und Epiphysis bei Nagethieren, Zeitschr. f. wissensch. Zoologie, 1884.

Kupffer, Primäre Metamerie der Neuralrohrs der Vertebraten, München. Sitzungsb., Bd. xv. McClure, The primitive segmentation of the vertebrate brain, Zool. Anzeiger, xii.

Marshall, A. Milnes, On the early stages of development of the nerves in birds, Journal of Anatomy and Physiology, 1877; The development of the cranial nerves in the chick, Quarterly Journal of Microsc. Science, 1878; On the head cavities and associated nerves of elasmobranchs, Quarterly Journal of Microsc. Science, 1881.

Mihalkovics, v., Wirbelsaite u. Hirnanhang, Archiv f. mikr. Anatomie, 1875; Entwicklungsgeschichte des Gehirns, Leipzig, 1877.

Onodi, A. D., Ueber die Entwicklung der Spinalganglien und der Nervenwurzeln, Internat. Monatsschr. f. Anat. u. Histologie, i., 3, 1884; Ueber die Entwicklung des sympathischen Nervensystems, Arch. f. mikr. Anat., Bd. xxvi., 1885.

Orr, H., Note on the development of Amphibians, chiefly concerning the central nervous system, &c. Quarterly Journal of Micr. Science, xxix., 1889.

Osborn, The origin of the corpus callosum, Morph. Jahrbuch, 1887.

Paterson, A. M., On the fate of the muscle-plate, and the development of the spinal nerves and limb plexuses in birds and mammals, Quarterly Journal of Micr. Science, Aug., 1887; The development of the sympathetic nervous system in mammals. Proc. Roy. Soc., April, 1890.

Rabl, C., Bemerkung über die Segmentirung des Hirns, Zoolog. Anz., 1885.

Rabl-Ruckard, Gehirn der Knochenfische, Arch. f. Anat. u. Physiol., Anat. Abth. 1882 und

1883.

Robinson, A., On the development of the posterior columns, of the posterior fissure, and of the central canal of the spinal cord. Owens' College Studies, 1890.

Rüdinger, Ueber die Bildung der Augenblasen, Sitzungsb. der Gesellsch. f. Morphol. zu München,

1889.

Spencer, B., On the presence and structure of the pineal eye in Lacertilia, Quarterly Journal of Micr. Science, 1886.

Strahl, H., und Martin, E., Die Entwicklung des Parietalauges bei Anguis fragilis und Lacerta vivipara, Arch. f. Anat. u. Phys., Anat. Abth. 1888.

Vignal, W., Sur le développement des éléments de la moelle des mammifères, Archives de physiol. 1884; Recherches sur le développement de la substance corticale du cerveau et du cervelet, Archives de physiologie, 1888.

Wijhe, J. W. van, Ueber Somiten und Nerven im Kopfe von Vögel- und Reptilienembryonen, Zoolog. Anzeiger, 1886.

Zuckerkandl, E., Ueber das Riechcentrum, Stuttgart, 1887.

DEVELOPMENT OF THE EYE.

The first development of the eye occurs as a hollow protrusion of the anterior cerebral vesicle-primary optic vesicle-in the manner already mentioned (see fig. 68, e, and fig. 93). The vesicle thus formed abuts externally against the external epiblast of the side of the head (fig. 96); and this external epiblast opposite the most prominent point of the primary optic vesicle, becomes thickened and in

vaginated, so as to form at first a hollow cup-shaped depression with thickened walls (figs. 97, 98), and subsequently by the closing in of the epiblast at the mouth of the cup, a hollow island of epithelial cells (fig. 99). This island, which is the rudimentary lens, lies between, but is entirely distinct from the external epiblast on the one hand, and the neural epiblast of the primary optic vesicle on the other hand. Its formation is accompanied by a cupping in of the primary optic vesicle (figs. 94, 97, 98), which is invaginated before it, and this invagination is increased by an ingrowth of mesoblast, which occurs between the lens and the cupped optic vesicle, and which subsequently forms the vitreous humour. Invaginated in this way the cavity of the original optic vesicle becomes almost entirely obliterated, and appears merely as a cleft between the two layers which form the wall of the so-called "optic cup." The inner of these two layers is from the first thicker than the outer, and in it are developed all the parts of the future retina from the membrana limitans interna

to the layer of rods and cones, while from the outer thinner layer the hexagonal pigmented epithelium of the retina, with its continuation into the uvea, is formed. The invagination of the primary optic vesicle does not occur only opposite the

Fig. 98. VERTICAL SECTION THROUGH THE MIDDLE OF THE DEVELOPING EYE OF A CHICK OF THE THIRD DAY. (E. A. S.)

The section passes longitudinally through the deficiency in the lower part of the optic cup, and shows the mesoblast extending in between the lens invagination and the pigment layer of the optic cup. thal, thalamencephalon; n.ep, neural epiblast; c.ep, cutaneous epiblast; o.s., optic stalk; 0, 0, cavity of primary optic vesicle; me, mesoblast; v, mesoblast passing behind lens to form vitreous; 7, lens invagination.

Fig. 99. SECTION THROUGH THE EYE AND OPTIC STALK OF A HUMAN EMBRYO OF FIVE WEEKS. (His.) W.c., connection of optic stalk with thalamencephalon; Sp, cleft or fold in the stalk, where the arteria centralis retinæ passes in; P, pigment layer; R, retina; L, lens.

place where the lens is becoming involuted, but also below, or ventral, to that place, so that a section exactly through the middle of the optic cup at right angles to the axis of this part of the head, shows a gap in the boundary of the cup through which the mesoblast is passing into the space between the lens and the invaginated optic vesicle (fig. 98, v). This gap or cleft soon becomes closed, but the suture or line of closure long remains apparent from the fact that when pigment begins to be deposited in the eye, this so-called choroidal fissure remains for some time unpigmented (until the sixth week in man).

The ventral invagination is in mammals continued for a considerable distance into the stalk of the optic vesicle (fig. 95), and the simultaneous inclosure of mesoblastic tissue leads to the introduction of the central blood-vessels of the retina within the optic nerve. In birds no such infolding of the stalk occurs.

The lower invagination of the optic cup serves not only to permit of the passage of mesoblast behind the lens for the formation of vitreous humour, but also to establish a direct connection between the nerve-fibres which are formed along the course of the optic stalk (future optic nerve) and the centre of the inner layer of the optic cup (future retina) (0. Hertwig).

The malformation termed coloboma iridis is attributed to a persistence of the choroidal cleft, which extends behind the iris along with the retinal pigment or uvea, as far as the margin of the pupil.

Fig. 100.-HORIZONTAL SECTION THROUGH THE EYE

OF AN EMBRYO RABBIT OF TWELVE DAYS AND SIX

HOURS. 70. (Kölliker.)

o, optic stalk; h', remains of the cavity of the primary optic vesicle; p, proximal lamella of the optic cup (pigmentum nigrum); r, distal lamella (retina); 7, lens invagination, widely open at ol; ', papillar elevation in the bottom of the lens vesicle; m, meso blast; g, mesoblast of vitreous; v, a blood-vessel at the anterior border of the optic cup; e, cutaneous epiblast.

Fig. 101.-EYEBALL OF A HUMAN EMBRYO OF FOUR WEEKS CUT ACROSS, AND THE ANTERIOR HALF REPRESENTED FROM BEHIND. (Kölliker.) 100. pr, the remains of the cavity of the primary optic vesicle; P, outer layer forming the retinal pigment; r, the thickened inner part giving rise to the columnar and other structures of the retina; , commencing vitreous humour within the optic cup; v', the cleft through which a vascular loop, a, projects from below; 1, the lens with a central cavity.

The hollow optic stalks are at first freely in communication with the thalamencephalon, or third ventricle. Nerve-fibres grow along their walls, from neuroblasts which develop in the retinal epiblast, and pass towards the nerve-centre (His), and the cavities of the stalks become thereby gradually obliterated, the radially striated epithelial-like arrangement of the wall being, however, long evident. A new connection becomes subsequently established between the posterior part of the optic stalks (optic tracts) and the mesencephalon, whilst the middle parts become united with one another to form the chiasma.

The development of the retina from the inner layer of the optic cup, has not been fully worked out. In its earlier stages it closely resembles in structure the wall of the cerebral vesicles, consisting of elongated epithelium-like cells, apparently arranged in several interlocking layers. Of these cells some become developed into nerve-fibres and nerve-cells (inner granules and ganglionic layer), others into sustentacular tissue, similar to the neuroglia of the central nervous system (molecular layers, Müllerian fibres), whilst the outermost layer forms the sense-epithelium (W. Müller), or layer of outer granules, which is sharply marked off against the layer of hexagonal pigment cells by the membrana limitans externa, as is the nervefibre layer from the vitreous humour by the membrana limitans interna. For a long time there is no trace of the rods and cones. These begin to appear some little time before birth in man and most animals, but in animals which are born blind, such as kittens, not until after birth (M. Schultze), in the shape of small protuberances of the sense-epithelium cells growing beyond the limitans externa, and forming at first the inner segments of the rods and cones, and subsequently the outer segments also. The latter as they are developed become imbedded in the inner surface of the hexagonal pigment cells, which have become developed from the outer layer of the optic cup.

The anterior third of the optic cup does not undergo the changes above

Fig. 102.-SECTION THROUGH THE EYE OF A RABBIT EMBRYO, MORE ADVANCED IN DEVELOPMENT THAN THAT SHOWN IN FIG. 98. (Balfour.)

c, epithelium of cornea; 1, lens; me.c, mesoblast growing in to form the substantia propria of the cornea; o.n, optic nerve; rt, retina; a.c.r, mesoblast for the formation of the vitreous humour, and the arteria centralis retina.

described. Its two layers become here developed into the comparatively simple pars ciliaris retinæ, and in front of the ciliary region they extend forwards and inwards in front of the lens and in close contact with the back of the iris, where they form the thickly pigmented epithelium, which is known as the uvea and terminates at the margin of the pupil.

Further development of the lens. The hollow epiblastic vesicle from which the lens develops is composed of a thick posterior and a thin anterior layer which pass into one another at the equator of the lens, and enclose a clear fluid. In mammals, the vesicle when first formed also contains a small mass of epithelium cells which have become separated off from the posterior wall (fig. 100), but these