206 EMBRYOLOGY 
transform the groove into a tube, beginning at the head end, while 
the posterior arms of the medullary folds remain asunder, and take 
the front end of the primitive streak between them. In the mean- 
time the cleavage of the lateral or more marginal portions of the 
FB. 
\ 
Pr.S. 
EMBRYO OF THE ComMMoN Fowt, 
FROM THIRTY TO THIRTY-SIX HOURS OF 
INCUBATION, FROM ABOVE. Magnified 
fifteen times. (After Balfour.) 
F.B. MB, HB, Vesicles of the fore-, 
mid-, and hind-brain; 0.V, Optic 
vesicle; Au.P, Auditory Pit; A.P. 
Area pellucida; Pv.V, Protovertebree 
(mesoblastic somites); MC, Medullary 
canal in region of sacral rhanboid 
sinus; Pr.S. Rest of the primitive 
streak ; T.F’. Tail-fold of the amnion. 
opaca has been spreading over 
the whole blastoderm at the end 
attained the size of a sixpeuce. 
mesoblast into an outer and an inner 
layer has taken place, the cleft being 
the future pleuro-peritoneal cavity (¢f. 
p. 203). This cleavage does not ex- 
tend throughout the whole of the 
mesoblast, but stops at some little 
distance to the right and left from 
the medullary groove and the noto- 
chord. These uncleft mesoblastic por- 
tions are called the vertebral plates, 
in opposition to the lateral plates or 
split portions of the mesoblast. At 
first the right and left vertebral plates 
remain unbroken along their length, 
but soon transverse constrictions ap- 
pear in them, and cut them up into 
a series of cubical masses. These 
cubes are called the mesoblastic som- 
ites or protovertebre ; they are the 
basis out of which the voluntary mus- 
cles of the trunk and limbs, and the 
bodies of the vertebrae are formed. 
The first pair of somites rises in level 
of the anterior end of the primitive 
streak, the next pairs are added on 
between the first and the streak, so 
that the latter seems continually car- 
ried back, the lengthening of the em- 
bryo always taking place between the 
front end of the streak and the last 
somite. All that part of the embryo 
which is in front of the first somite 
corresponds to the future head, and 
the rest to the neck, body, and tail. 
During all these changes the area 
the surface of the yolk, so far that 
of the first day of incubation, has 
Vessels appear in the area opaca 
in the immediate neighbourhood of the embryo. 
Changes during the 2nd day.—During the first half of this the 
medullary folds are closing r 
the neural canal, closed in front, but still open behind. 
apidly ; the groove is converted into 
The portion 
