18 GENERAL EMBRYOLOGY. 
Structure of the Ectoderm and Entoderm.—The cells of the ectoderm are at 
first irregular in size and shape, and their outlines are indistinct ; but after a short 
time the ectoderm cells at one pole of the blastodermic vesicle become cubical or 
shghtly columnar, whilst the remaining cells of the outer layer are flattened and 
have irregular outlines. 
The columnar cells 
form the ectoderm of 
the embryo, and the 
flattened cells are util- 
ised in the formation of 
aT, nutritive and protec- 
streak tive structures known 
as the placenta and 
foetal membranes. 
Mesoderm The cells of the en- 
toderm are also, at first, 
very irregular in shape 
and size, but after- 
wards, as they are 
A, Before the appearance of the primitive streak—the embryonic area is spread out into a layer, 
circular in form and bilaminar throughout ; B, After the appearance of they become more or 
the primitive streak. The posterior end of the primitive streak shows less rounded, and they 
a crescentic thickening, which indicates the commencement of the meso- 
derm or middle layer of the blastodermic membrane. 
e 
Fic. 11.—SuRFacE VIEW OF THE BLASTODERMIC VESICLE. 
Showing the embryonic area and the commencement of the mesoderm. 
Embryonic 
area 
A 
anastomose together by 
filamentous processes. 
At a still later period they are transformed into polygonal plates which appear 
spindle-shaped in section (Figs. 10 and 12). 
The Embryonic Area.—When the upper pole of the bilaminar blastodermic 
vesicle is examined in surface view. from above, a dark, somewhat opaque circular 
area is visible; this is known as the embryonic area. It is coextensive with the 
columnar portion of the ectoderm. Very soon after 1t appears the embryonic area 
becomes ovoid; the small end of the ovoid area is posterior, that is, it lies in the 
region which is afterwards converted into the posterior part of the embryo. At 
A 
Embryonic area 
— Neural groove 
Embryonic B —— 
area —~ 
: 3 -Ectoderm 
Neural 
groove - ] 
Entoderm 
Primitive Embryonic area 
streak I nae 
Cc | Primitive groove 
Ectoderm 
Mesoderm Mesoderm 
Entoderm 
Fic. 12.—THE Upper POLE OF THE BLASTODERMIC VESICLE. 
Showing the embryonic area, the primitive streak with the extension of the mesoderm from its sides and 
posterior end, and the commencement of the neural groove. 
A, Surface view (diagrammatic) ; B and C, Transverse sections through the blastoderm of the ferret at the 
stage represented in A and along the lines 4 and c respectively. 
the hinder end of the ovoid area a still darker patch of triangular form is developed ; 
this soon becomes crescentic, and is the first indication of the primitive streak and 
of the formation of a third blastodermic layer termed the mesoderm or mesoblast. 
The primitive streak consists of thickened ectoderm which is seen in transverse 
sections projecting downwards, and resting upon the entoderm, in the form of a 
ridge. 
From the sides and the postericr extremity of the ectodermal ridge a lamina of 
