LETTERING FOR ALL FIGURES ON PLATES VI.-XXVIII.

a, head-fold of amnion.
aa, anterior appendage.
ac, anterior cardinal vein.
al, allantois.
an, anterior nares.
ao, aorta.
aop, area opaca.
ap, area pellucida.
ar, aortic arch.
au, auricle.
b, bulbus arteriosus.
bc, body cavity.
blp, blastopore.
bp, basilar plate.
bv, blood-vessel.
c, centrum of vertebra.
ca, caudal artery.
ch, cerebral hemisphere.
cl, cloaca.
cn, cranial nerve.
cp, posterior choroid plexus.
cv, cardinal veins.
dc, ductus Cuvieri.
e, eye.
ec, ectoderm.
ec′, thickening of ectoderm,
en, entoderm.
en′, endocardium.
ent, enteron.
ep, epidermal layer of ectoderm.
epi, paraphysis.
es, embryonic shield.
f, fronto-nasal process.
fb, forebrain.
fg, foregut.
g¹⁻⁵, gill clefts.
gf¹⁻⁶, gill folds.
gl, glomerulus.
h, head-fold.
hb, hindbrain.
ht, heart.
i, intestine.
i′, stomach.
in, infundibulum.
ir, iris.
it, iter.
k, kidney (metanephros).
l, remains of groove between secondary folds.
la, larynx (cartilages of).
li, liver.
ll, lower lid of eye.
ln, lens.
lr, inferior rectus muscle of eye.
lu, lungs.
lv, lens vesicle.
m, mouth.
ma, manus.
mb, midbrain.
mc, medullary canal.
me′, tip end of medullary canal.
md, mandibular fold.
mes, mesoderm.
mes′, myocardium.
mf, medullary fold.
mg, medullary groove.
mk, Meckel’s cartilage.
mp, muscle plate.
ms, mesentery.
mv, meatus venosus.
mx, maxillary fold.
myc, myocœl.
n, nasal invagination or cavity.
na, neural arch of vertebra.
nc, neurenteric canal.
nl, nervous layer of ectoderm.
nt, notochord.
o, ear vesicle.
oc, optic cup.
oe, œsophagus.
on, optic nerve.
os, optic stalk.
ov, optic vesicle.
p, pituitary body.
pa, posterior appendage.
pan, pancreas.
pc, posterior cardinal vein.
pe, pes.
pg, primitive groove.
ph, pharynx.
pl, pelvis.
pn, posterior nares.
pr, pericardial cavity.
ps, primitive streak.
pt, pecten.
rt, retina.
s, somites.
sc, spinal cord.
se, sphenethmoid cartilage.
sf, secondary fold.
sg, spinal ganglion.
sm, splanchnic mesoblast.
sn, spinal nerve.
so, somatic mesoblast.
st, stomodæum.
sy, sympathetic nervous system.
t, tail.
ta, trachea.
tg, thyroid gland.
th, thickening and posterior limit of sf.
tn, tongue.
to, tooth anlage.
tr, truncus arteriosus.
tv, third ventricle of brain.
tv′, third ventricle of brain.
u, umbilical stalk.
ul, upper lid of eye.
ur, superior rectus muscle of eye.
v′-″-‴, first, second, and third cerebral vesicles.
va, vascular area.
vm, vitelline membrane.
vn, ventricle of heart.
vv, vitelline blood-vessels.
wd, Wolffian duct.
wdo, opening of Wolffian duct.
wr, Wolffian ridge.
wt, Wolffian tubules.
y, yolk.