962 



OF GENERATION EMBRYONIC DEVELOPMENT. 



spondence between the progressive stages of development of the Human Cere- 

 brum, and those which we encounter in the ascending series of Mammalia. 1 

 799. The development of the Eye 1 commences by a protrusion from the 

 lower and posterior part of the anterior cerebral vesicle, the cavity of the 

 protrusion being continuous with that of the vesicle itself; very soon, how- 



FIG. 364. 



Longitudinal section of the Eye of Embryo Fowl : 1, from an embryo at about the 65th hour of in- 

 cubation ; 2, from an embryo a few hours older; 3, from an embryo at the 4th day of incubation: h, 

 epiblast; I, lens in Fig. 1, still connected with the corneal lamina, and possessing a small cavity (o) in 

 its interior, in Figs. 2 and 3 it is seen detached but still hollow ; r, introverted portion of the primitive 

 optic vesicle, subsequently becoming the retina ; a, posterior part of the optic vesicle, which, according 

 to Remak, probably becomes the choroid coat, ciliary processes, and iris, and in Figs. 1 and 2 is still 

 connected with the brain by the hollow optic nerve ; x, thickening of the epiblast around the spot froui 

 which the lens has detached itself; gl, vitreous body. 



ever, it loses all connection with the Cerebral Hemispheres, and opens ex- 

 clusively into the " third ventricle." As soon as the primitive optic vesicle 

 has assumed its permanent position, the Epiblast over it becomes thickened 

 (Fig. 364, 1 ), whilst the subjacent wall of the optic vesicle becomes indented, 

 and at length so far pushed in as to form a sac composed of two layers, the 

 edges of which surround the lens (Fig. 364, 2). This sac is the "secondary 

 optic vesicle" or "cup." The anterior thicker layer (Fig. 364, 3, r) is de- 

 veloped into the retina, the posterior (a) becomes the hexagonal tessellated 

 pigment layer of the choroid. The doubling in of the primary optic vesicle 

 is so conducted that a slit remains for a short time, termed the choroidal 

 fissure, through which a portion of mesoblast gains entrance into the interior 

 of the vesicle. The thickened layer of the epiblast (e) becoming detached 

 constitutes the lens, a thin layer of the epiblast still remaining, which forms 

 the epithelium of the cornea. The mesoblast surrounding the optic vesicle 

 gradually forms a distinct investment for it, of which the outer part consti- 

 tutes the sclerotic and cornea, and the inner the proper tissues and vessels of 

 the choroid, whilst another portion penetrating more deeply constitutes the 

 suspensory ligament and capsule of the lens, and the vitreous. The choroid 

 only reaches as far as the edge of the lens at first, and no trace of Iris can 

 at that time be discovered. At the end of the second month the iris, at 

 first represented by a colorless lamina in which pigment is afterwards de- 

 posited, grows out from the edge of the choroid, and insinuates itself be- 

 tween the lens and the cornea. The development of the anterior half of 

 the Retina ceases during the latter part of fetal life, and this part is gradu- 

 ally transformed into the Pars ciliaris, which in the adult does not contain 

 any nervous element. The Macula lutea is not present in the embryo, nor 



1 See an account of the observations of Prof. Ret/ins on the Development of the 

 Cerebrum, in the Archives d'Anatomio Generate et de Physiologic, 1846 



2 Sec tin- excellent Memoir of the late Mr H. Gray in the Philosophical Trans- 

 lations, 1H")0; Kollikcr, Kntwickel. des Mensehen, 1861, p. 273; Peter 011112:, On 

 tin' Development of the Eye in the Chick, Mcd.-Chir. Rev., 1858. vol. xxii, p. 187; 

 and Klcbs, in Virchow's Archiv, Bd. xxviii. 



