DEVELOPMENT OF THE CHICK SECOND PERIOD. 313 



anterior cerebral cell, at the present epoch, is symmetrical, 

 and contains the hemispheres (figs. 339, 341, and 345 b, 

 340 g, 344 d, 343 B, p] ; according to the natural curvature 

 of the embryo, it lies completely downwards. The optic nerve 

 appears as a vesicle, betwixt the middle and anterior cerebral 

 cell, in which the external envelopes (the outer portion of the 

 serous membrane), preparatory to the formation of the eye ball, 

 bend circularly inwards, in the shape of a sac, and externally 

 form a projection, which opens downwards as a cleft ; this is 

 closed by degrees, and at length forms a colourless thin streak, 

 whilst the rest of the bulb, from the deposition of the pigmen- 

 tum nigrum, is dark or deeply coloured ; the lens makes its 

 appearance very early (on the third day), forming a particular 

 closed capsule within the sac of the external envelopes (the 

 ball of the eye), and lying in the midst of an albuminous 

 ball, the vitreous humour.* The organ of hearing, at first a 

 simple vesicle arising from the medulla oblongata, soon be- 

 comes a distinct sac, which, examined from behind, appears 

 attached to the medulla oblongata by means of a pedicle the 

 acoustic nerve (fig. 340, i) ; distinct from it a cleft appears 

 (fig. 342, A,/), which increases over against the acoustic sac, 

 and sinking into it, forms the external meatus audjorius. If 

 the embryo be lying upon its side, the acoustic sac, which 

 subsequently forms the labyrinth, is seen as a rounded en- 

 largement (figs. 339 g, 341 e, 342 A, n), which in the course 

 of the period under consideration, comes continually forward. 

 About the beginning of the third day, the olfactory nerve 

 shows itself towards the basis of the cell of the hemispheres ; 

 at a later period the nasal holloiv (fig. 341, c) is observed as a 

 broad depression with puffed edges ; on the fifth day both 

 nasal hollows have become deeper, and are now distinct from 

 one another. 



489. Very important metamorphoses go on during this 

 period in the ventral laminae lying on either side of the dorsal 

 laminae, or middle portion of the embryo ; so far these ventrai 

 laminae are formed from the serous layer of the germinal mem- 

 brane only ; they separate into a superficial thinner layer (figs. 

 338 and 343, A, 5 2 and /), which, like a cuticle, loses itself 

 in the periphery of the embryo upon the deeper stratum ; and, 

 as it has already suffered a reflection anteriorly opposite the 



* On the metamorphosis of the eye, consult figs, from 339 to 310. 



