864 DEVELOPMENT [CH. LTX. 



convolutions appear in quick succession ; first the great primary ones 

 are sketched out, then the secondary ones. The commissures of the 



Fio. 662. Longitudinal section of the primary optic vesicle in the chick, magnified (from Remak). A, 

 from an embryo of sixty-five hours ; B, a few hours later ; C, of the fourth day ; c, the corneous 

 layer or epidermis, presenting in A the open depression for the lens, which is closed in B and C ; 

 I, the lens follicle and lens ; pr, the primary optic vesicle ; in A and B, the pedicle which forms 

 the optic nerve is shown ; in C, the section being to the side of the pedicle, the latter is not shown ; 

 v, the secondary optic vesicle and vitreous humour. 



brain, and the corpus eallosum, are developed by the growth of fibres 

 across the middle line. 



The Hippocampus major is formed by the folding in of the grey 

 matter from the exterior into the lateral ventricles. 



The Bye. Soon after the first three cerebral vesicles have 

 become distinct from each other, the anterior one sends out a lateral 



op 



Fio, 668. Diagrammatic sketch of a vertical 

 longitudinal section through the eyeball of a 

 human foetus of four weeks. The section is a 

 little to the side, so as to avoid passing 

 through the ocular cleft ; c, the cuticle where 

 it becomes later the corneal epithelium ; I, 

 the lens ; op, optic nerve formed by the 

 pedicle of the primary optic vesicle ; vp, 

 primary medullary cavity or optic vesicle ; p, 

 the pigment layer of the retina ; r, the inner 

 wall forming the remaining layers of the 

 retina ; vs, secondary optic vesicle containing 

 the rudiment of the vitreous humour, x 100. 

 (Kb'lliker.) 



Fio. 664. Transverse vertical section of 

 the eyeball of a human embryo of 

 four weeks. 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 retinal pig- 

 ment; r, the thickened inner part 

 giving rise to the other structures of 

 the retina ; v, the commencing vitre- 

 ous humour within the secondary 

 optic vesicle ; v', the ocular cleft 

 through which the loop of the central 

 blood-vessel, a, projects from below ; 

 I, the lens with a central cavity, x 

 100. (Kolliker.) 



vesicle from each side (primary optic vesicle), which grows out 

 towards the free surface, its cavity communicating with that of the 



