v.] ^ THE LENS-CAPSULE. 109 



grows forward over the front of the lens, the latter receives 

 a complete mesoblastic investment. 



Of this mesoblast a very thin layer next to the lens 

 both in front and behind becomes separated from the 

 rest, and forms the lens-capsule and suspensory ligament. 

 The remainder of the mesoblast behind the lens becomes 

 converted into the vitreous humour, the layer immediately 

 in contact with the retina giving rise to the hyaloid 

 membrane. That the hyaloid is really a product of the 

 mesoblast and not a cuticular outgrowth from the epiblastic 

 cells of the retina is indicated by the fact that it is con- 

 tinuous over the pecten, where of course the retina is absent. 

 At its first appearance the vitreous humour is a mass of 

 stellate cells ; while however it is rapidly enlarging to fill 

 up the growing optic cup, a large portion of it becomes 

 entirely fiuid, the cellular elements being more and more 

 restricted to the immediate neighbourhood of the posterior 

 surface of the lens, where a few stellate cells may be seen 

 even in the adult. 



Briefly to recapitulate. The eye commences as a lateral 

 outgrowth of the fore-brain, in the form of a stalked vesicle. 



The stalk becoming narrowed and subsequently solid, is 

 converted into the optic nerve. 



An involution of the superficial epiblast over the front of 

 the optic vesicle, in the form first of a pit, then of a closed sac 

 with thick walls, and lastly, of a solid rounded mass (the small 

 central cavitj^ being entirely obliterated by the thickening of 

 the hind wall), gives rise to the lens. Owing to this involu- 

 tion of the lens, the optic vesicle is doubled up on itself, and its 

 cavity obliterated ; thus a secondary optic vesicle or optic cup 

 with a thick anterior and a thin posterior wall is produced. As 

 a result of the manner in which the doubling up takes place, 

 or of the mode of growth afterwards, the cup of the secondary 

 optic vesicle is at first imperfect along its under surface, where 

 a gap, the choroidal fissure, exists for some little time, but 

 subsequently closes up. 



The mesoblast in which the eye is imbedded gathers 

 itself together around the optic cup into a distinct invest- 

 ment of which the internal layers become the choroid, the 

 external the sclerotiq. An ingrowth of this investment 

 between the front surface of the ler^s and the superficial 



