Monthly Microscopical"] 
Journal, Nov. 1, 1869. J 
Histology of the Eye. 
233 
it with the exterior cutis, and nutrient blood-vessels enter the 
vitreous humour through this channel. At a later stage, the foetal 
cleft closes, which perfectly isolates the internal corpus vitreum 
from the external cutis. Yon Ammon says that the closure of the 
foetal cleft begins at its middle, and proceeds hence in both direc- 
tions, forwards and backwards. 
Simultaneously with the transformation of the primary eye- 
vesicle into the secondary, the hollow eye-stalk became solid by the 
approximation of the upper and lower plates, and acquired the form 
of a flat ribbon. Next, by the inbending of its edges, the ribbon 
became a gutter, along which the blood-vessels gained the inside of 
the eye ; and, lastly, the gutter, closing in the eye-stalk, takes the 
cylindrical form of the perfect optic nerve, and includes the blood- 
vessels within it. 
Our knowledge of the distribution of these vessels is still very 
imperfect. Yon Ammon, whose articles in the ' Archiv fur Oph- 
thalmolgie' are a fund of information on the embryology of the 
eye, says that the arieria centralis, immediately on entering the 
globe, gives off fine twigs to the sclerotic and choroid ; next it de- 
taches several lateral tranches to the retina, upon the inner surface 
of which they spread out and form the memhrana vasculosa foetalis 
retinse ; then it sends off a second set of lateral branches, from five 
to seven in number, which ramify on the outer surface of the 
hyaloid capsule, forming here the discus arteriosus hyaloideus ; 
and, finally, the diminished trunk, traversing a canal in the vitreous 
humour, is distributed to the vascular capsule of the lens. Thus 
Yon Ammon describes two vascular nets — one retinal, the other 
belonging to the vitreous humour ; but this has not been confirmed 
by later observers. The late H. Miiller distinctly says that there 
are not any other vessels on the outer surface of the corpus vitreum 
than the retinal ones; and he also mentions that the retina con- 
tinues long without l3lood-vessels — a fact which I have myself 
verified in the human foetus, the moment of their appearance being 
apparently determined by that of the obliteration of the arteria 
hyaloidea capsules lentis. In the human foetus of the fifth month, 
in which all the retinal layers except the bacillary were distinctly 
recognizable, I found the retina still quite devoid of blood-vessels ; 
the axial vessels going to the lens-capsule were still pervious ; and 
I failed to detect the vascular net on the hyaloid capsule described 
by Yon Ammon. 
Absolutely fresh human embryos are so rarely obtainable that 
the structure of the human vitreous humour in the earliest stages 
of development is unknown. Before and after the fifth month 
it consists of a web of delicate fibres, the meshes of which contain 
a viscid colourless substance. Throughout this tissue, in chromic 
acid preparations, numerous minute bright globules occur, which, 
