THE EMBRYO. 27 
appendages, and of the glands which open on it. Thus the hairs and hair-follicles, the 
nails, the enamel of the teeth, the epithelium of the sebaceous glands, of the sweat 
glands, and of the mam- : 
mary glands are all ecto- 
dermal. The epithelium of 
the conjunctive and of the 
lachrymal glands is also de- 
rived from ectoderm. The 
roof of the mouth, the inner — M 
surfaces of the cheeks, 
-the nasal passages and 
their associated cavities, 
together with the adjacent 
part of the pharynx and VI 
the anterior lobe of the I 
pituitary body, as well as 
SC SC 
Fic. 20.—THE RELATIVE POSITIONS OF THE BLASTODERMIC LAYERS 
the external auditory canal IN THE Bopy OF THE EMBRYO WHEN THE “FOLDING OFF” Is 
and the outer layer of the CompLeTED (Diagrammatic). 
tympanic membrane, are Aly Transverse section through the umbilical aperture. 
IJ. Similar section in front of or behind the umbilicus. 
all developed from the sur- 
face, and their epithelium, AC, Alimentary canal. EN. Entoderm. N. Notochord. 
an ; : EC. Eetoderm. M. Mesoderm. SC. Spinal cord. 
with that of their glands, VI. Vitello-intestinal duct and umbilical aperture. 
is ectodermal in origin. 
The epithelium of the sense organs, except that of taste (the tongue), is derived 
from ectoderm; the auditory and olfactory epithelial elements, and those of the 
lens and cornea, are from surface ectoderm ; whilst the epithelial elements of the 
retina are from neural ectoderm. 
The neural ectoderm is removed from the surface to form the neural tube and 
neural crest, from which the cells and fibres of the whole of the nervous system, 
Fic. 21.—TRANSVERSE SECTION OF A FERRET EMBRYO, 
Showing further differentiation of the mesoderm. 
CC. Central canal. ML, Muscular layer of protover- SC. Spinal cord. 
CL. Cutaneous lamella of protover- tebral somite. SG. Spinal ganglion. 
tebral somite. N. Notochord. SL.  Scleratogenous layer of pro- 
CO. Celom. NC. Neural crest. tovertebral somite. 
EC. Ectoderm. PA. Primitive aorta. SoM. Somatic mesoderm. 
EN. Entoderm. PS. Protovertebral somite. SoP. Somatopleure. 
GC. Germinal cell. SB. Spongioblast. SpM. Splanchnie mesoderm. 
SpP. Splanchnopleure. 
both central and peripheral, and the sustentacular tissue of the brain and spinal 
cord, are developed. The neural ectoderm also furnishes the epithelial elements of 
the retin, the pineal gland, and of the posterior lobe of the pituitary body. 
The entoderm lines the alimentary canal and the spaces and glands which open 
cells receive their nourishment from blood-vessels which are in their vicinity. Epithelial cells are 
modified in accordance with the particular functions they are called upon to serve, and they present many 
variations in shape, size, and structure, ¢.g. the neuro-epithelial cells of the central nervous system and of 
the peripheral sense organs differ considerably from the more ordinary epithelial type; but they are 
simply more specialised, and therefore more modified. 
