DEVELOPMENT OF THE FROG, 
601 
Internally, a differentiation of hypoblast forms the notochord 
along the mid-dorsal line of the 
archenteron, At each side of 
this lie masses of mesoblast which 
have been split off from the hypo- 
blast. Each of these divides into 
the primitive segments (proto- 
vertebre) above, and the un- 
segmented lateral plates below. 
The lateral plates split into two 
layers, the splanchnic or inner 
investing the gut, the somatic or 
outer layer being applied to the 
epiblast ; the space between the 
two layers is the body cavity. 
The body now becomes dis- 
tinctly divided into regions, the 
eyes bud out from the brain, a 
rudiment of the gills appears, 
and the larva, still within its 
gelatinous case, exhibits peculiar 
lashing movements of the tail. 
~ Eventually, about a fortnight 
after the eggs are laid, the larva 
escapes from the surrounding 
jelly and swims in the water. At 
this stage and for some time the 
ectoderm is ciliated. There is a 
cloacal opening, but the mouth is 
not yet more thana dimple. A 
glandular crescent, often mis- 
named a sucker, lies on the 
under surface of the head, and 
secretes a sticky slime, by means 
of which the tadpole attaches 
itself to foreign objects. The 
protruding gills soon become 
branched. -There are three of 
them -on each side, the first the 
largest. They are covered with 
Fic. 326. — Dissection of 
tadpole. — After Milnes 
Marshall and Bles. 
DL., Lower lip; #., ventricle of 
heart; DZ., cesophagus; 1VA., 
head kidney; A.; aorta; K., 
kidney; AU., ureter; DO., 
cloaca; ZLH., hind-limb; XV., 
opening of ureter into cloaca >. 
GR., genifal ridge; GF., fatty 
body; L/., fore-limb; OG., gills ; 
a, epidermis ; 4, dermis. 
ectoderm, and are borne on the outside of the first 
