26 GENERAL EMBRYOLOGY. 
many rudiments of the embryo become distinguishable at an early period in its 
development, e.g. the embryonic area, the primitive streak and groove, the neural 
groove, the notochord and the protovertebral somites, the body of the embryo does 
not assume its characteristic form until it becomes raised and folded off from the 
general surface of the blastodermic vesicle. 
The main cause of the folding off of the embryo from the surface of the vesicle 
is the more rapid growth of the embry onic area as contrasted with the slower growth 
and expansion of the remainder of the wall of the vesicle; and the moulding of the 
increasing embryonic area into the form of the embryo is due to differences in the 
rate of erowth of the various parts of the area itself. 
The manner in which the area is folded, and the changes in the relative _ 
positions of its various parts which necessarily result, will be facilitated by reference 
to Figs. 19 and 25. 
The embryonic area at an early period increases rapidly, especially in length. 
Its margins, however, appear to remain comparatively fixed, and hence as the area 
increases it must fold upon itself. It becomes somewhat convex externally, and is 
raised slightly above the general surface, but at the same time it apparently sinks 
into the interior of the ovum, and the amnion folds close over it. 
The antero-posterior growth is greater than the lateral; consequently the 
folding of the embryonic area is most ‘marked in front and behind. Anterior and 
posterior, or cephalic and caudal, folds are formed, which indicate the head and 
tail extremities of the embryo. Similarly, lateral folds define the lateral limits of 
the body. 
When the body of the embryo thus becomes folded off it contains a portion of 
the blastodermic cavity and of the ccelom; the former is the primitive alimentary 
canal, and the latter is the rudiment of ‘the pericardial, pleural, and peritoneal 
cavities. 
The communication between the pleuro-peritoneal and the corresponding 
extra-embryonic cavity is obviously bounded by the margins of the embryonic area, 
and it constitutes the umbilical orifice. 
The margins of the embryonic area retain approximately their original positions, 
and in its further growth the embryo extends beyond them in all directions. 
THE EMBRYO 
The embryo, now easily distinguishable from the rest of the ovum, is already 
sufficiently developed to give some indication of the general plan of its organisation 
and of the ultimate relation and fate of the three layers of the blastoderm which 
enter into its constitution. There are as yet no limbs, but the general contour of 
the head and body are defined. It possesses a notochord, afterwards replaced by 
the permanent vertebral column, which constitutes a longitudinal central axis. 
On the dorsal aspect of the notochord the neural groove is closing to form the 
neural canal, or primitive cerebro-spinal nervous system, whilst, on its ventral side, 
a portion of the blastodermic cavity is being included as a primitive tubular 
alimentary canal, which freely communicates with the remainder of the blastodermie 
cavity, now ¢ alent the cavity of the yolk-sac. 
The formation of the protovertebral somites, which is the first indication of that 
seementation which is such a characteristic feature in the structure of the vertebrate 
body, has commenced. 
The general relations of the three layers of the blastoderm remain unaltered. 
Thus, externally, there is a layer of ectoderm forming the surface of the body ; 
internally, a layer of entoderm lining the primitive alimentary canal; and between 
them is the mesoderm, enclosing the ccelom. 
The surface ectoderm forms the epithelial elements! of the skin and _ its 
1 The term ‘‘ Epithelium ” is applied to tissues consisting of cells which are united with one another 
by means of a small amount of intercellular substance. 
The cells constituting epithelium are always arranged in one or more layers ; they cover free surfaces 
and line the various cavities of the body, including the vascular and lymphatic systems ; they also form 
the active elements in secretory glands and line their duets. Epithelium is always non- vascular, and the 
