194 MR. H. GRAY ON THE DEVELOPMENT OF THE RETINA AND OPTIC NERVE, 
I shall proceed, in the next place, to consider the development of the various layers 
of the retina. 
Structure of the Retina of the Chick at the Eighth Day of Incubation. 
The retina may at this period be distinctly separated as a thin transparent layer 
from the other membranes. Its choroidal surface appears to be composed of a closely 
aggregated mass of globular nuclei ; these bodies are about the size of the red cor- 
puscles of the blood, and form about one-half of the entire thickness of the mem- 
brane ; they apparently correspond to the ‘‘agglomerated granules” mentioned by 
Bowman as forming a considerable portion of the membrane in the normal state ; 
they are highly refractive and of a slight yellow tinge. The deep surface of the 
membrane consists of some fine granular matter, and a mass of pale and exceedingly 
delicate nucleated cells, precisely similar to those surrounding the meshes of the 
fibrous lamina in the normal structure of the membrane. No trace at this period ex- 
isted of either the membrana Jacobi, or the fibrous lamina of the retina. 
On the Development of the Membrana Jacobi. 
Between the thirteenth and fourteenth days the choroidal surface of the retina pre- 
sents an exceedingly fine pale granular stratum, which covers in the “ agglomerated 
granular mass ” beneath. On the fifteenth day numerous exceedingly minute and 
highly refractive yellowish granules are imbedded in it ; they vary in size, between 
the 5000th and the 8000th part of an inch in diameter, and around some of these a 
fine delicate cell-wall can be traced ; if a good view can now be obtained of this sur- 
face, it will present the appearance of a delicate epithelial layer. This I believe to be 
the first stage in the development of Jacob’s membrane ; for on the eighteenth day the 
cells, which were previously of a circular form, had now become elongated, some 
being of an oval shape, whilst in others the almost perfect rod-like body was formed ; 
they now lie in a slightly imbricated manner, and their nuclei, which are of a bright 
yellow colour, are placed at the apices of the rods ; in some cases however they occupy 
the middle of these bodies ; their deep ends are larger than their choroidal ends, and 
are strongly attached by this surface to the “agglomerated granular layer” beneath ; 
so intimate is their connexion that it is difficult to get a good view of a single rod in 
the field whilst examining them, probably on account of their perfect separation 
from this layer not having yet taken place. On the twenty-first day Jacob’s mem- 
brane is similar to what is seen in the full-grown bird ; the rods are now closely 
packed together, standing perpendicularly upwards, and all of an elongated cylin- 
drical form, their tips being occupied by the brilliant yellow bodies before noticed. 
On the Development of the Fibrous Lamina. 
The first trace of the fibrous lamina is seen between the fourteenth and fifteenth 
days : on examining the deep surface of the retina at this period, it is seen to be com- 
