In the Ammocœte the choroid slit is merely represented by a slight notch on the ventral edge of the optic cup, and the mesoblastic process which passes through the choroid slit in most types is represented by a large cellular process, from which the vitreous humour would appear to be derived.
Mammalia differ from all the types already described in the immense fœtal development of the blood-vessels of the vitreous humour. There are however some points in connection with the development of these vessels which are still uncertain. The most important of these points concerns the presence of a prolongation of the mesoblast around the eye into the cavity of the vitreous humour. It is maintained by Lieberkühn, Arnold, Kölliker, etc., that in the invagination of the lens a thin layer of mesoblast is carried before it; and is thus transported into the cavity of the vitreous humour. This is denied by Kessler, but the layer is so clearly figured by the above embryologists, that the existence of it in some Mammalia (the Rabbit, etc.) must I think be accepted.
In the folding in of the optic vesicle, which accompanies the formation of the lens, the optic nerve becomes included, and on the development of the cavity of the vitreous humour an artery, running in the fold of the optic nerve, passes through the choroid slit into the cavity of the vitreous humour ([fig. 295], acr). The sides of the optic nerve subsequently bend over, and completely envelope this artery, which at a later period gives off branches to the retina, and becomes known as the arteria centralis retinæ. It is homologous with the arterial limb of the vascular loop projecting into the vitreous humour in Birds, Lizards, Teleostei, etc.
Fig. 295. Section through the eye of a Rabbit embryo of about twelve days.
c. epithelium of cornea; l. lens; mec. mesoblast growing in from the side to form the cornea; rt. retina; a.c.r. arteria centralis retinæ; of.n. optic nerve.
The figure shews (1) the absence at this stage of mesoblast between the lens and the epiblast: the interval between the two has however been made too great; (2) the arteria centralis retinæ forming the vascular capsule of the lens and continuous with vascular structures round the edges of the optic cup.
Before becoming enveloped in the optic nerve this artery is continued through the vitreous humour ([fig. 295]), and when it comes in close proximity to the lens it divides into a number of radiating branches, which pass round the edge of the lens, and form a vascular sheath which is prolonged so as to cover the anterior wall of the lens. In front of the lens they anastomose with vessels, coming from the iris, many of which are venous ([fig. 295])—and the whole of the blood from the arteria centralis is carried away by these veins. The vascular sheath surrounding the lens receives the name of the membrana capsulo-pupillaris. The posterior part of it appears (Kessler, No. [372]) to be formed of vessels without the addition of any other structures and is either formed simply by branches of the arteria centralis, or out of the mesoblast cells involuted with the lens. The anterior part of the vascular sheath is however inclosed in a very delicate membrane, the membrana pupillaris, continuous at the sides with the epithelium of Descemet’s membrane. On the formation of the iris this membrane lies superficially to it, and forms a kind of continuation of the mesoblast of the iris over the front of the lens.
The origin of this membrane is much disputed. By Kessler, whose statements have been in the main followed, it is believed to appear comparatively late as an ingrowth of the stroma of the iris; while Kölliker believes it to be derived from a mesoblastic ingrowth between the front wall of the lens and the epiblast. According to Kölliker this ingrowth subsequently becomes split into two laminæ, one of which forms the cornea, and the other the anterior part of the vascular sheath of the lens with its membrana pupillaris. Between the two appears the aqueous humour.
The membrana capsulo-pupillaris is simply a provisional embryonic structure, subserving the nutrition of the lens. The time of its disappearance varies somewhat for the different Mammalia in which this point has been investigated. In the human embryo it lasts from the second to the seventh month and sometimes longer. As a rule it is completely absorbed at the time of birth. The absorption of the anterior part commences in the centre and proceeds outwards.
In addition to the vessels of the vascular capsule round the lens, there arise from the arteria centralis retinæ, just after its exit from the optic nerve, in many forms (Dog, Cat, Calf, Sheep, Rabbit, Man) provisional vascular branches which extend themselves in the posterior part of the vitreous humour. Near the ciliary end of the vitreous humour they anastomose with the vessels of the membrana capsulo-pupillaris.