604 



GENERATION AND DEVELOPMENT. 



pressing on the front of this process or primary optic vesicle, is 

 a pouch of the common integument, which subsequently be- 

 comes a shut sac, and in which is developed the lens and its 

 capsule (Fig. 236). Subsequently there is protruded from 

 below upwards, between the lens in front and the primary 

 optic vesicle behind, another process or pouch, remaining for 

 some time imperfect below, and called the secondary optic vesicle. 

 The deficiency below contracts into what is called the ocular 

 cleft, which subsequently becomes entirely obliterated. In 

 connection with the primary optic vesicle are developed the 



FIG. 237. 



op 



FIG. 236. Diagrammatic sketch of a vertical longitudinal section through the eye- 

 ball of a human foetus of fourweeks (after Kolliker) !!. The section is alittle to the 

 side, so as to avoid passing through the ocular cleft ; c, the cuticle where it becomes 

 later the cornea; I, the lens; op, optic nerve formed by the pedicle of the pri- 

 mary optic vesicle ; vp, primary medullary cavity or optic vesicle ; p, the pigment 

 layer of the choroid coat of the outer wall ; r, the inner wall forming the retina; vs, 

 secondary optic vesicle containing the rudiment of the vitreous humor. 



FIG. 237. Transverse vertical section of the eyeball of a human embryo of four 

 weeks (from Kolliker) 1 o o. The anterior half of the section is represented : pr, the 

 remains of the cavity of the primary optic vesicle ; p, the inner part of the outer 

 layer forming the choroidal pigment; r, the thickened inner part giving rise to 

 the columnar and other structures of the retina; v, the commencing vitreous 

 humor within the secondary optic vesicle ; v', the ocular cleft through which the 

 loop of the central bloodvessel, a, projects from below ; I, the lens with a central 

 cavity. 



retina from the invaginated portion, and the pigmentary 

 portion of the choroid in connection with the outer part (Fig. 

 236). In the secondary optic vesicle the vitreous humor is 

 formed. The outer walls of the eyeball, the sclerotic and 

 cornea, are developed from the tissues immediately around 

 those which have been just described. 



The iris is formed rather late, as a circular septum project- 



