176 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



(2) Neuroblast, which forms (a) the optic nerves, (b) sensitive 

 retina, (c) pars ciliaris retinae, (d) uvea (e) pigmentary layer of 

 retina. 



(3) Mesoblast, which forms (a) outer tunic (sclerotic and 

 fibrous cornea) ; (b) middle tunic (choroid, ciliary- choroid and 

 iris) ; (c) the vitreous humour and its capsule — the hyaloid mem- 

 brane ; (d) the capsule of the lens. 



1. Structures derived from the Epiblast. — (a) The lens.— 

 The lens is developed by a saccular invagination of the epiblast 

 situated over the optic vesicle (Kg. 142). It becomes a closed 



epiblast. 

 lens, uesic. 



optio vesicle 



epiblast 

 lens, vesic. 



mesobl. 



uesicle 



optio cup. 



Fig. 142. Fia. 143. 



Fig. 142. — Invagination of the Epiblast to form the Lena Vesicle. 

 Fig. 143. — The manner in which the Lens Vesicle is severed from the Epiblast. 



sac by the severance of its connection with the epiblast, its wall 

 being formed by a single layer of epithelial cells. The cavity of 



lens uesicle 



mesoblast (capsule) 

 lens fibres 



epiblast (cornea) 



Fig. 144.— The Formation of the Lens Fibres from the Epithelium on the posterior 

 Wall of the Vesicle. 



the lenticular vesicle is gradually obliterated by the cells of the 

 posterior wall becoming elongated (Fig. 144) until they reach 



