THE EMBRYONIC AREA. 19 
cells projects outwards, and gradually insinuates itself between the ectoderm and 
the entoderm over the whole area of the vesicle, except in certain regions to be 
afterwards described. This lamina is the rudiment of the mesoderm. With the 
formation of the mesoderm the blastodermic membrane becomes trilaminar. 
The majority of the cells of the mesoderm are derived from those of the 
primitive streak, but it is said that cells from the entoderm also take part in its 
formation. Young mesodermal cells are round or ovoid, and some give off numerous 
processes. In later stages they may assume various shapes, and many closely 
resemble the cells of the ectoderm or those of the entoderm. 
As the blastodermic vesicle grows, the embryonic or germinal area becomes 
pyriform and increases in length, principally in the posterior part of its extent 
where the primitive streak is situated; at the same time the streak lengthens and 
becomes more linear. For a short time a groove, the primitive groove, appears on 
the surface of the streak. It is deepest in front, where in some mammals, includ- 
ing man, a small transitory perforation is formed, the neurenteric canal. 
A second broader and shallower groove then appears in the embryonic area 
immediately in front of the primitive streak; this is the neural groove, the rudiment 
of the nervous system. The neural groove, its bounding folds, and the nervous 
NHC 
a age 
<a _ SoM 
OCA +s, Seer 
8 oe eae ee * jee 
PM 
N 
Fic. 138.—TRANSVERSE SECTION OF A FERRET EMBRYO. 
Showing neural groove before the separation of the paraxial from the lateral mesoderm. 
C.  Celom. GC. Germinal cell. PM. Paraxial mesoderm. SoM. Somatic mesoderm, 
EC. Kctoderm. N. Notochord. SB. Spongioblast. SoP. Somatopleure. 
EN. Entoderm. NG, Neural groove. SG. Spinal ganglion. SpM. Splanchnie mesoderm, 
SpP. Splanchnopleure. 
system subsequently developed from them are formed entirely of ectodermal elements, 
which at first are continuous with those forming the outer layer of the embryo. 
The posterior end of the neural groove embraces the anterior end of the primitive 
streak and groove, and at this period the neurenteric canal forms a communication 
between the interior of the ovum and the bottom of the neural groove, which 
latter afterwards becomes the closed canal of the central nervous system. In some 
vertebrates the neurenteric canal persists for a considerable period, and upon the 
development of the alimentary canal it constitutes a communicating channel 
between it and the cavity of the neural tube. 
As the neural groove grows backwards the anterior part of the primitive streak 
is absorbed, and although the posterior part continues to grow, the primitive streak 
as a whole diminishes in length; ultimately the greater part of the primitive 
streak disappears, but a portion is recognisable for a considerable time extending 
from the base of the tail, a transitory structure in the human embryo, to the 
ventral wall of the body. This portion forms the posterior boundary of the 
primitive alimentary canal; it remains bilaminar, and is called the cloacal 
membrane. 
The primitive streak is of great morphological importance ; recent researches have shown 
that from it and the cells in its neighbourhood practically the whole of the body of the embryo, 
with the exception of the anterior part of the head and heart region, is developed. It probably 
represents the mouth of a remote (pre-vertebrate) ancestor, the fused lips of which formed the 
body of a primitive vertebrate animal. The aperture of this mouth is still represented in lower 
vertebrates by an opening known as the blastopore. The neurenteric canal is the only represen- 
tative of the opening in the human subject. 
The neural or medullary groove is bounded laterally by medullary folds 
