580 



DEVELOPMENT OF THE 



[sect. 230. 



of the epidermis, during the formation of the lens, and thus comes 

 to lie within the eye. The vitreous body may, then, be looked 

 upon in the light of undeveloped subcutaneous connective tissue ; 

 and the more so since all the subcutaneous connective tissue of 

 the embryo is at one period perfectly gelatinous (as I have shown in 

 § 29 of this work, 3rd Germ, ed.) ; again, the enamel organ, which 

 also belongs to this category, bears a striking resemblance to the 

 mucous tissue of the vitreous body, both in appearance and con- 

 sistence. 



With regard to the histological development of the eye, the follow- 

 ing remarks only require to be made here. At an early period, the 

 eye consists in all its parts of homogeneous formative-cells, which 



in the course of time become transformed 

 into the different tissues. In the fibrous coat, 

 the cells become converted, in the second 

 and third months, into connective tissue, in 

 the manner before described (§ 26); and at 

 the same time, the cornea and sclerotic ex- 

 hibit a distinction, they being at first quite 

 the same in external appearance, and consti- 

 tuting only a single coat. In the uvea, the 

 cells are mostly employed in the formation 

 of the vessels ; another part of them is con- 

 verted into the inner and outer pigment 

 layers, pigment granules being deposited at 

 the commencement of the third month; 

 while a third part develops into the muscles, 

 nerves, epithelial and connective tissues of 

 this coat. In the retina, the development of 

 the nerve-cells and of the so-called 'granules' 

 from the embryonic cells, can be readily 

 traced. I have observed the cones to de- 

 velop in the same way ; and in the frog I 

 believe I am warranted in assuming that the 

 rods are nothing but elongated cells (see my 

 Micr. Anat., ii., 2, p. 730) ; on the other 

 hand, the formation of the rods in mam- 

 malia, and that of the nerve-fibres them- 

 selves, have not yet been followed. The 

 lens, lastly, consists at its origin entirely of 

 cells, which, in the course of time, pass into 

 The fibres of the lens of the foetus, and of that of the 



Fibres of the lens of an adult 

 in process of development ; 

 magnified 350 times. 1. a 

 young fibre with the nucleus 

 at the anterior extremity ; 

 seen from the surface and from 

 the side. 2. a similar fibre, in 

 ■which the elongation forwards 

 is commencing. 3. a rather 

 long fibre, elongated towards 

 both extremities ; a. its pos- 

 terior, 6. its anterior ex- 

 tremity. 



fibres. 





