Vertebrate Embryology

The Nervous System,

concerned with the relations of the individual to the outer world, develops, as might be expected, from the ectoderm. Normally its first rudiment in the embryo consists of a thickening of the ectoderm along the dorsal surface— the medullary plate. As development proceeds the edges of this curl upwards as the neural folds and these arching in towards one another, convert the originally flat plate first into a groove and later, by fusion of its edges in the mesial plane, into a closed neural tube. Of this, the portion in the head region becomes greatly enlarged to form the brain, while the remainder gives rise to the spinal cord. The brain was formerly described as differentiating in the form of three dilatations of the neural tube one behind the other, the so-called primary brain vesicles, giving rise to fore-, mid-, and hind-brain. Modern advances in compara tive embryology show that this is not accurate. In most of the more primitive holoblastic vertebrates the brain first becomes differentiated into an anterior cerebrum and a posterior rhom bencephalon, demarcated from one another by an upward fold of the brain floor. Of these the former becomes later differentiated into the thalamencephalon in front and the mesencephalon be hind. The cerebral hemispheres, which in the higher mammals assume great importance, seem to arise primitively as paired outward bulgings of the side-wall of the brain towards its front end—related to the sense of smell. The main part of the thala mencephalon undergoes thickening of its side-walls (optic thala mus) while its roof becomes for the most part degenerate, form ing a thin membrane in intimate contact with a network of blood vessels (choroid plexus) lying immediately outside it. At its hind end an outgrowth—the pineal body—develops which may remain a simple club-shaped or tubular structure, but in several cases becomes differentiated into two portions, the anterior of which (parapineal) develops into an eye (Sphenodon, various lizards). In front of the fold of the brain-floor already alluded to, the floor of the thalamencephalon dips down as the infundibulum, and this becomes in the course of development closely associated with an independent structure—the pituitary body. In the more typi cal vertebrates the two become inextricably involved with one another and it is customary to speak of the nervous part of the pituitary body. The pituitary ingrowth of the ectoderm is typi cally a hollow pocket and in the surviving crossopterygians it re tains this form through life, forming a gland which opens into the buccal cavity. In those vertebrates in which yolk is present at the site of its formation, the pituitary ingrowth is, as in other such cases, solid, developing its cavity secondarily.

The organs of special sense arise as localized developments of the ectoderm. In the case of the olfactory organ and the audi tory organ, the rudiment shows first as a localized thickening of the ectoderm, which then, through extension in area, becomes in voluted below the surface of the skin as a saucer-like depression. Finally, the opening to the exterior becoming gradually con stricted, the organ assumes the form of a more or less completely closed vesicle. In the case of the olfactory organ the closure is never complete, the function, that of chemical testing of the surrounding medium, necessitating free communication between its cavity, in the lining of which the sensory cells develop, and the outside. In the majority of vertebrates however partial closure takes place, to divide the opening into two—one at each end of the organ—and so render possible the drawing in of a current of the external medium through the organ. The first vertebrates that have this power of are the lungfishes and the origin of the arrangement which makes this possible is well seen in Protopterus, where the opening of the olfactory organ narrows, except at its two ends, so as to form a slit. The edges of the slit then undergo fusion and the original single opening is now repre sented by two separate openings a considerable distance apart.

As the anterior boundary of the buccal cavity becomes delimited, one, the anterior or external naris, is left outside and the other, posterior or internal naris, is enclosed within the buccal cavity, perforating its roof. In terrestrial vertebrates in general the olfactory organ becomes similarly provided with external and internal nares, though the process of development shows various modifications in detail.

The early stages of development of the otocyst or rudiment of the auditory organ, are similar to those of the olfactory organ, but the reduction of the external opening goes further : in fact in all vertebrates except elasmobranchs, it becomes completely closed. The peculiar feature which distinguishes the vertebrate is that the usually pyriform otocyst of early stages undergoes a complicated process of modelling, whereby its wall comes to pro ject into three hollow ridges situated in planes perpendicular to one another. The basal or attached portion of each of these be coming obliterated except at its two ends, the ridge is converted into an arched tube—the semicircular canal—opening at each end into the cavity of the otocyst and filled, like the rest of the oto cyst, with watery endolymph. In all except the most archaic vertebrates, the otocyst undergoes a still further process of modelling whereby its lower portion (saccule), which develops a special pocket-like outgrowth devoted to the sense of hearing, becomes more or less completely constricted off from the upper portion or utricle, carrying the semicircular canals.

The vertebrate eye differs from the other sense-organs in that its main portion—that containing the actual sensory cells—is developed, not from the external ectoderm, but from the involuted portion of the ectoderm which forms the brain. In a typical case, as in a bird embryo, the optic rudiment consists in its earliest stage simply of the lateral portion of the wall of the thalamencephalon, which here extends outwards on each side so as to give the brain a T-shape. As development proceeds the optic rudiment becomes narrowed at its base to form the optic stalk, which later will become the optic nerve. The distal dilated portion gives rise to the retina, while the region of external ectoderm in contact with its outer end gives rise to the lens. In a typical case, e.g., a bird, the lens is at first simply a slight thickening of the ectoderm, but this soon sinks inwards to form a saucer-shaped depression of the surface, which, by a gradual narrowing of its opening, becomes converted into a closed vesicle. The deep wall of this becomes greatly thickened, its individual cells becoming tall and columnar, and gradually takes on the form of a biconvex lens, the outer wall forming a thin layer of epithelium covering its outer surface. As development goes on the cells of the lens become keratinized and transparent Meanwhile the original optic rudiment is undergoing differentia tion. Its distal portion next the lens becomes involuted within the proximal portion, so that the whole rudiment now takes the form of a double-walled optic cup the mouth of which is blocked by the lens. The inner wall of the cup increases much in thick ness and gradually assumes the immense complexity character istic of the functional retina, of which the most striking peculiarity is that the visual cells are situated on its deep face, the sensory rods facing not towards the lens but away from it—so that rays of light have to traverse the whole thickness of the retina, which is therefore necessarily transparent. On the other hand, the nerve fibres which pass from the retina to the brain emerge from the retina on its face next the lens, instead of from its deep face as one would expect. This extraordinary reversal of the vertebral retina is at once explained by the method of its development, the deep surface carrying the rods having been originally, before the involution of the brain-rudiment took place, part of the outer surface of the head.