Royal Society, 547 



similar to those found surrounding the fibrous lamina in the normal 

 structure of the membrane. 



The " Membrana Jacobi " is first observed on the thirteenth day 

 as a fine pale granular stratum which covers in the globular nuclei 

 already described. In this, at about the fifteenth day, some brilliant 

 yellowish granules are imbedded ; they vary in size from the 5000th 

 to the SOOOth of an inch, and around them a delicate cell-wall is 

 traced ; they soon acquire an oval shape ; then become more elon- 

 gated ; and about the eighteenth day the almost perfect rods are 

 formed. They are now disposed in an imbricated manner, and their 

 nuclei, which are of a bright yellow colour, are placed generally at 

 the apex, but sometimes in the middle of the rods. On the twenty- 

 first day this membrane is similar to what is seen in the full-grown 

 bird. 



The first trace of the " fibrous lamina" is seen between the four- 

 teenth and fifteenth days, as a fine pale granular lamina marked by 

 numerous faint longitudinal striae. On the eighteenth day this 

 membrane when separated from the other layers is seen composed 

 of numerous fibrillated meshes, in which are deposited the nucleated 

 vesicles which are formed as early as the eighth day. From these 

 observations it is seen that the retina is formed as a protrusion from 

 the most anterior cerebral cell, being hollow and communicating with 

 its cavity ; that it subsequently assumes a pyriform shape, presenting 

 a dilated end, the future retina, and a tubular portion, the optic 

 nerve. As the tubular portion becomes solidified so as to form the 

 optic nerve, then no communication can be traced between the op- 

 tic vesicle and the cavity from which it is an offset. By degrees 

 the spherical end of the protrusion is absorbed, and the retina, being 

 now fully formed, becomes attached to the margin of the lens. The 

 optic nerve is then traced to be connected not only with the anterior 

 cerebral cell, but, uniting with its fellow at the under surface of the 

 optic lobes, is seen partly to terminate in those bodies. The deduc- 

 tions from these observations may be thus briefly stated : — 



1st. They confirm the observations on the structure of the retina 

 made by Bowman, who has shown that the essential part of this 

 membrane is analogous to the cineritious matter of the brain, and is 

 composed like it of a fibrous mesh intermingled with vesicles of 

 grey matter, being, in fact, a portion of the cerebrum pushed out- 

 wards and connected with the brain by its appropriate commissure, 

 the optic nerve. The mode of development of this membrane 

 would show this to be the correct view of the structure of this 

 essential part of the retinal expansion, and at the same time disprove 

 the statements of Henle, who believed it to be more analogous to 

 epithelium. 



2nd. The origin of the optic vesicle from the anterior cerebral 

 cell, would show the incorrectness of the opinion of those anato- 

 mists who have stated that none of the fibres of the optic nerve 

 could be traced to the optic thalami. The thalami being developed 

 from the same centre from whence these vesicles arise, would render 

 it exceedingly probable that the optic nerves had some connection 

 with those bodies. 



