THE EYES. 129 



matter and the cells dorsal to it become continuous with 

 the same structures on the opposite side (PI. Xj fig. 2). A 

 little farther back the connection between the two brain lobes 

 is effected only by the white matter, the dorso-median patch 

 of cells entirely disappearing (PI. X, fig. 3). A few sec- 

 tions farther back the latter again appear, and their appear- 

 ance is soon followed, in the section series, by the separation 

 of the cerebral lobes (PL X, fig. 4). 



The ventral appendages of the brain have already been 

 dealt with (p. 127). 



The Eyes. — At the close of Stage e the eyes are closed 

 vesicles, connected by their ventral corners with the brain 

 immediately external to the white matter (PI. VII, fig. 22 a). 

 The connection, which is at first a broad one, has in Stage g 

 become constricted to a narrow pedicle — the optic nerve — 

 connecting the inner wall of the optic vesicle with the brain 

 (PI. X, fig. 1) ; at the same time the nuclei withdraw from 

 the optic nerve, and from the portions of the wall of the 

 optic vesicle and of the brain, which lie next the optic nerve 

 (PI. X, fig. 1). In this manner the white matter of the so-called 

 optic ganglion of the adult is established, and the optic nerve 

 comes to consist entirely of white maiter. The layer which 

 will form the rods in the adult eye appears at a very early 

 stage as a result of the withdrawal of the nuclei of the thick 

 inner wall of the vesicle from the internal surface (PI. VII, 

 fig. 22 a, and PI. X, fig. 1, rods). The pigment and lens 

 have not yet been formed. The eye, therefore, in Stage g 

 (PI. X, fig. I) consists of a vesicle with a thin outei- wall 

 closely subjacent to the epidermis, which is very thin over the 

 eye, and of a thick inner wall lying close to and connected with 

 the brain by a cord of nerve-fibres. The inner wall further 

 presents a patch of white matter at the point of entrance of 

 the optic nerve, and a layer of white matter (the rods) next the 

 lumen of the vesicle. The pigment appears in January at the 

 junction of the layer of rudimentary rods and the nuclei. The 

 lens is formed at about the same time, as a secretion of the 

 wall of the vesicle. It lies within and fills up the cavity ol 



