DEVELOPMENT OF THE RETINA. 203 



compressed into columnar fasciculi, which remain stretched 

 between the two limitantes, or between the limitans interna 

 and the external granulated layer. Though it was originally 

 described by H. Muller* under the impression that the infiltra- 

 tion was a post-mortem change, it was first depicted by 

 Blessig,f and first stated to be of frequent occurrence by Henle.J 

 When transverse sections of such cedematous spots of the retina 

 are made, cavities are seen in the region of the granule layers, 

 or there may be extensive degeneration of the tissues between 

 the limitans externa and interna,. bounded by closely com- 

 pressed columns of radial fibres, in which are many nuclei, and 

 which form arches near the limiting membranes. Degenera- 

 tions of this kind, however, are not exclusively limited to the 

 ora serrata. I have myself observed a case where a portion of 

 the retina, the size of a pea, was so infiltrated as to form a pro- 

 minent tumour near the equator of the eye. Transverse section 

 showed that there was a well-marked state of cedema confined 

 to this spot. The retina was here about one millimeter in thick- 

 ness. The rods and cones which, in minor degrees of oedema, 

 appear to be unaltered disappear in the more completely dis- 

 tended parts. Merkel also observed cedematous swelling of 

 the retina in old Dogs. 



6. DEVELOPMENT OF THE RETINA. 



To form the retina a vesicle is protruded from the brain of 

 the embryo, termed the primitive eye vesicle, which very soon 

 after its appearance becomes changed with coincident develop- 

 ment of the lens into a doubly laminated cup. This occurs in 

 Fowls at the end of the second day of incubation. The two 

 laminae of the primitive retina which originates from the eye 

 vesicle are, in the first instance, of equal thickness ; but the 

 anterior lamina, which is in contact with the vitreous, soon 



* Zeitschrift fur wissenschaft. Zoologie, Band viii., p. 71. 

 t De Retinae, structura, fig. 3, p. 47. Dorpat, 1855. 

 Eingeweidelehre, p. 669. 

 See Iwanoff, loc. cit., Taf. iv. and v., figs. 11 and 12. 



