Cuap. II. FOLDINGS OF THE EMBRYONIC DISC. 577 
noid plexus has become a large and dense bunch of bloodvessels (fig. 4, 4a, 6, w-c. 
3, k, w-e. 4, k, w-c. 6, b, 6’) hanging in the cavity just below the cerebellum. The 
Se spinal marrow (fig. 4, 5, w-ce. 5, % w-c. 5, a) has 
Hears eb eat seapetne 8 closed over, close up to the medulla oblongata, (fig. 
ae 4, 6, w-e. 3, 7’, w-e. 6, ¢, ¢,) but the latter remains 
broadly open. The whole surface of the brain, 
above and below, is covered rdodee 8: 
by a delicate membrane, filled «¢ » + wv d 
by a dense network of blood- 
vessels (fig. 4, 5, 6, 7, 8, 9, 
10, w-c. 3, 9’ g”, w-e. 5, d, w-e. 
6, d, w-e. 7, h, w-e. 10; -e): 
this is the pia mater. The 

cells of the tissue of the 
olfactory nerve are beginning 

to unite with each other in a linear series (Pl. 19, fig. 15) 
to form nervous tubes. The tissue cells of the base of 
this nerve, close to the olfactory bulb, are sharply polyg- 
onal (Pl. 19, fig. 17, 17a, 17b) and elongated. The cells of the cerebrum are 
not so regularly polygonal as at the base of the olfactory nerve, but still 
mq WwW DiAgon 
retain more or less of their original rounded contours (EL19; Woodseut 9: 
fig. 16, e, 16a). The eyes are (Pl. 21, fig. 33, 33a; Pl 22, , 
fig. 8, 8a) perfectly developed, as far as the requirements of , :. 

Wood-cut 10. sight are concerned, excepting some feat- 
a is: ures of secondary importance: the bony 
ring (Pl. 22, fig. 7, ¢) in the sclerotica, . 
(%,) and the double membrana pupil- 
laris (n, m1) before the capsule of the 
lens, (i,) in the adult, are not yet apparent. (See fig. 8, 4, 
the sclerotica, and x, 7, the membrana pupillaris.) The ears 
(Pl. 23, fig. 6, 7, we. 6, ¢ ¢, w-e. 7, 4 ff) — Woodcut 11. 
have nearly as complicated a labyrinth as 
in the adult. 
The amount of ossification of the bones 

is very unequal in different parts of the 
body. The vertebral column (PI. 23, fig. 4, w-e. 3, g) and the 
bones of the sternum are the most advanced in this respect, two 
thirds of the bone at least being hardened, mostly next the sur- 
face. The bones of the upper jaw (Pl. 23, fig. 4, fig. 11, we 3, 4 we H, ¢, 
73 

