242 BULLETIN OF THE UNITED STATES FISH COMMISSION. 
fi 
is shown by the presence of the infundibulum. The latter in this stage has barely V, 
begun to grow backwards. Its composition of long columnar cells needs no description, i' 
Fig. 135, PI. CIV, is through the anterior part of the eyes. The third ventricle here 
sends up a dorsal process, presumably towards what will become the pineal gland. ' 
Ventrally the optic nerves (op. n.) are met with. The most peculiar feature in the ( 
embryonic brain is clearly the great forward extension (Fig. 134) of the thin roof, 
characteristic of the fourth ventricle. The condition of the spinal cord at the time of 
hatching is sufficiently indicated in the Figs. 119 and 126, PI. cii. Pig. 127, PI. cm, ' 
representing transverse sections from the extreme posterior end forwards. 
The histological differentiation into nerve cells and fibers begins both in the spinal 
cord and brain shortly (about 12 hours) after hatching. A peripheral accumulation 
of fibrous matter is formed, and some of the peripheral cells abandon their simple 
elongated embryonic shape, and assume the appearance of rounded nerve cells, having- 
in general a single process (Fig. 136, PI. civ, spinal cord). The transformation of ' 
the elongated embryonic cells into rounded nerve elements proceeds rapidly, and on | 
the second day after hatching there are no embryonic cells to be seen (Fig. 139, Plate | 
cv, larva of 112 hours). During the histological transformation of the cells the canal 1 
of the spinal cord also loses its embryonic shape. The successive stages in the meta- | 
morphosis of the canal are shown in Fig. 136, PL Civ, and Figs. 139, 141, and 143, PL j 
cv. The closure progresses gradually from the edges towards the center. The dis- I 
tribution of the fibrous matter on the fourth day after hatching (larva 160 hours) is I 
shown in Fig. 143, PL ov. It is only on the third or fourth day after hatching | 
that the spinal nerve roots can be made out (Figs. 141 and 143, d. n. r., v. n. r.), and 
then they are so very small that their presence alone can be demonstrated. So 
with most of the cranial nerves, their minute size renders it impossible to follow their 
development. ' 
Histological differentiation of the surface ectoderm . — The thin membranous ectoderm ' 
of the non-embryonic area has already been mentioned. It is composed of the epidermic 
stratum, and one or two strata of flattened cells. At the end of embryonic life, when 
the yolk sac begins to disappear, the ectoderm covering it gradually becomes thick- 
ened while the rest of the ectoderm grows thin (Figs. 139-141, PL cv). It is thus 
brought about that the yolk ectoderm in the larval stages is thicker than the rest of 
the layer. 
After the wide neural plate (axenplatte) has been transformed into a deep keel, the 
ectoderm of the embryo, except in the immediate neighborhood of sense organs, is 
made up of the epidermic stratum and two strata of ‘‘nervous layer” cells. Its con- 
dition just before the separation of the neural cord is shown in Fig. 81, PL xcvii, part 
of a section, such as Fig. 75, PL xcvii. The further development of the general ecto- | 
derm during embryonic life consists in the vacuolation of the outer stratum of the ! 
nervous layer (1, Fig. 81), and the flattening of the inner stratum (2, Fig. 81). The ' 
vacuolation has begun in Pig. 81. Almost all of the cells of the outer layer become 
vacuolated, and usually there is in each cell a single large vacuole, as is shown in Fig. 
112, PL Ci, a more highly magnified view of the ventral portion of a section such » 
as Fig. 111. Inside the vacuolated layer is the second stratum made up now of flat- 
tened cells. This condition of the ectoderm is indicated in the figures of the later 
embryonic stages, such as Fig. 126, PL cii. 
