24 GENERAL EMBRYOLOGY. 
the thickened portions are situated in the regions of the intervertebral discs, and 
the intermediate constricted portions in the regions of the vertebral bodies. The 
vertebral portions gradually disappear, and the intervertebral parts are converted 
into a kind of mucoid tissue, the pulp of the intervertebral discs. 
Formation of the Coelom.—Before the extension of the mesoderm over the 
blastoderm is completed, that 1s, before it has reached the ventral pole of the ovum, 
it begins to separate into two layers. The separation is brought about by cleft-like 
spaces which appear within its substance near the margin of the embryonic area, 
and, rapidly coalescing, form the ccelom or body cavity. Increasing in size, the 
ccelom extends towards both poles of the blastodermic vesicle, but in the embryonic 
area it is arrested before it quite reaches the notochord and the sides of the primitive 
streak. It extends across the pericardial area, however, and forms the rudiment of 
the pericardial cavity, which appears as a transverse tubular passage continuous 
on each side with the general body cavity. Thus the mesoderm is separated into 
two layers, except along the sides of the notochord and primitive streak. The 
outer or parietal of these layers becomes more or less closely attached to the 
ectoderm, and with it forms the somatopleure, whilst the inner or visceral layer 
is similarly associated with the entoderm to form the splanchnopleure. 
There is reason to believe that in man and other higher mammals the ectoderm 
and entoderm are separated by a large space before the formation of the mesoderm 
Dio mae commences, and that the meso- 
aree derm is cleft as it extends, 
the somatopleuric part grow- 
ing round the inner surface 
-Pericardial of the ectoderm and_ the 
Busse splanchnopleuric layer extend- 
pharyngeal —_ ing over the outer surface of 
the entoderm; therefore it 
may be said that the two layers 
of the mesodern extend from 
the primitive streak and grow 
Primitive 
cerebral 
dilatations 
Protovertebral 
poet Seite round a space which intervenes 
in the first instance between 
Embryonic : E 
aoe the ectoderm and the ento- 
derm, and which, when the 
yaxaot et, eXtension of the mesoderm is 
segmented) — Gompleted, becomes the ecelom. 
This mode of extension of 
the mesoderm is merely a 
modification of the more 
general plan of its separation 
Fia, 18.—Surrace View or AN Earty Empryo (Diagrammatic). into layers at a later period. 
Showing the neural groove, dilated in the head region but still un- When the cclom is fully 
closed, and the first protovertebral somites. The margins of the formed the blastoderm con- 
ccelomie space are indicated by dotted lines. ae Oye 
tains two cavities, one, the 
ccelom or body cavity, situated between the two layers of the mesoderm, and the 
other the cavity of the blastodermic vesicle, usually called the vitelline cavity, 
which lies inside the entoderm. 
Mesodermic or Protovertebral Somites.— During the formation of the coelom 
the undivided mesoderm at each side of the notochord thickens, principally by a 
dorsal upgrowth which is coincident with the uprising of the ectodermal medullary 
folds which bound the neural groove. There are thus formed two thickened bars 
of mesodermal tissue, one on each side of the neural tube, and they together con- 
stitute the paraxial mesoderm (Fig. 13); the more laterally situated portions of the 
mesoderm are known as the lateral plates. 
The paraxial mesoderm is soon divided, except in the head region, by a number 
of transverse clefts into a series of cubical masses termed the protovertebral 
somites. These are at first partially, and afterwards more completely separated 
from the lateral plates by longitudinal grooves. After the longitudinal grooves 
Rhomboida] 
sinus 
