CH. LVIII.] 



DEVELOPMENT OF THE EYE. 



847 



and man, they grow still further back, until they completely 

 cover in the cerebellum, so that on looking down on the brain 

 from above, the cerebellum is quite concealed from view. The 

 surface of the hemispheres is at first quite smooth, but as early as 

 the third month the great Sylvian fissure begins to be formed 

 (fig. 665). 



The next to appear is the parieto-occipital fissure ; these two 

 great fissures, unlike the rest of the sulci, are formed by a curving 

 round of the whole cerebral mass. 



In the sixth month the fissure of Rolando appears : from this 

 time till the end of foetal life the brain grows rapidly in size, and 

 the convolutions appear in quick succession * first the great pri- 

 mary ones are sketched out, then the secondary ones. The 



Fig. 666. 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 

 ocular vesicle and vitreous humour. 



commissures of the brain (anterior, middle, and posterior), and the 

 corpus callosum, 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 Eye. Soon after the first three cerebral vesicles have 

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

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

 out towards the free sxirface, its cavity communicating with that 

 of the cerebral vesicle through the canal in its pedicle. It 

 remains connected to the thalamencephalon. It is soon met and 

 invaginated by an in-growing process from the epiblast of the 

 surface (fig. 666). This process of the epiblast is at first a depres- 

 sion, which ultimately becomes closed in at the edges so as to 

 produce a hollow ball, which is thus completely severed from the 

 epidermis with which it was originally continuous. From this 



