12 Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. 
extends along the lateral line, from the posterior end of the branchial plate to a point midway between 
the cloaca and the tip of the tail. 
The main external differences between this embryo and the preceding one 11.5 mm. in length are: 
the greater expansion of the telencephalon and mesencephalon, and the change in shape of the somites. 
Figs. 26, 26a, 26b. Embryo 15.0 mm. in length. Normal plate series No. 26. H.E.C. 227, 228, 229. 
Lateral, dorsal, and ventral views. X 5. 
The head is bent a little ventrally from the hind-brain. The telencephalon is much expanded and 
extends forward a little over the region of the diencephalon. The epiphysis forms a small median rounded 
swelling between the telencephalon and mid-brain. The nasal pits are deep semicircular depressions on 
either side of the telencephalon. The optic cup is I.o mm. in diameter. The optic cleft is faintly visible. 
The roof of the hind-brain is semitransparent. Five gill slits open to the pharynx. The spiracular slit is 
the longest and lies almost parallel to the longitudinal axis of the body. The posterior borders of the 
first four branchial arches are prolonged a little backward over the clefts as gill flaps. Three short gill 
filaments project from each of the second, third and fourth arches. The posterior border of the branchial 
thickening is sharply defined. The mouth opening is almost as broad as long. The mandibular arches 
are bent at their middles at an angle of 90°; their bases meet at an angle of 100°. The heart cavity is 
expanded ventrally, and is a little constricted above. The anterior and posterior dorsal fins are indicated 
by elevations of the dorsal median fin fold. Each of these elevations is about I mm. in length. The 
pectoral fins lie immediately behind the branchial area or plate and over the yolk stalk. They are I.o mm. 
in length and project 0.5 mm. from the body. The posterior end of each is directed a little ventrally. 
A broad and shallow groove extends along the lateral line, from the pharyngeal region nearly to the end 
of the tail. There is a distinct groove between the ventral ends of the somites and the somatopleure below. 
The cloaca forms a prominent swelling on the ventral surface of the body, 5 mm. from the end of the tail. 
The noticeable external differences between this embryo and the preceding one 13.0 mm. in length 
are as follows: the whole head is bent a little ventrally from the body; the telencephalon and mesen- 
cephalon are considerably more expanded; the fifth gill slit is open; gill faps are developed on the four 
anterior gill arches; the dorsal fins are differentiated from the dorsal fin fold; the cloaca and the heart 
chamber are more prominent. 
Fig. 27. Embryo 18.0 mm. in length. Normal plate series No. 27. H.E.C. 203, 204, 205. Lateral 
view. X5. 
The head, from the posterior boundary of the hind-brain, is bent a little ventrally. The fore-brain 
is much expanded. The mid-brain is expanded downward as well as in front. The roof of the hind-brain 
is semitransparent. The optic cup is almost circular in outline; its greatest diameter is I.4 mm. The walls 
of the spiracular cleft are in contact. Gill filaments project from the anterior five clefts — those from the 
hyoid clefts are the longest. The anterior end of the pectoral fin is 0.5 mm. posterior to the branchial 
plate, and the main body of the fin lies posterior to the yolk stalk. The fin is approximately 2.0 mm. in 
length at its base. The elevation of the anterior dorsal fin is scarcely noticeable. The posterior dorsal fin 
is 0.5 mm. in height. 
The chief differences between this and the preceding embryo, 15.0 mm. in length, are as follows: 
the fore-brain is much more expanded and projects farther ventrally; the pelvic and posterior dorsal fins 
are larger; the gill flaps are farther developed, and the gill filaments are longer. 
m a 
