THE DEVELOPMENT OF THE EMBRYO ITSELF. 95 
The vertebral plates, when distinctly marked off, as repre- 
sented in the figure, are termed the protovertebre (mesoblastic 
somites), and represent the future vertebre and the voluntary 
muscles of the trunk; the former arising from the inner sub- 
divisions, and the latter from 
the outer (muscle-plates). It 5b. 
will be understood that the pro- 
tovertebree are the results of 
transverse division of the col- 
umns of mesoblast that formed ™”- 
the vertebral plates. hb. 
Before the permanent verte- 
bree are formed, a reunion of “= 
the original protovertebre takes 
place as one cartilaginous pillar, 
followed by anew segmentation 
midway between the original 
divisions. 
It thus appears that a large 
_number of structures either ap- 
pear or are clearly outlined dur- 
ing the first day of incubation: 
the primitive streak, primitive 
groove, medullary plates and 
groove, the neural canal, the 
head-fold, the cleavage of the 
mesoblast, the protovertebree, Fic. 107.—Embryo of chick, between thirty 
. & and thirty-six hours, viewed from above 
with traces of the amnion and as an opaque object (Foster and Balfour). 
=== sncaaeGl 
. b, forebrain ; m. 6b, midbrain; h. b, 
area opaca, oe i Op. v, optic vesicle ; au. p, 
i auditory pit; o. f, vitelline vein; p. v 
During the second day near- mesoblastic ‘atte : m. f, line of func: 
aos . tion of medullary folds above medulla 
ly all the remaining important canal; s.r, sinus rhomboidalis; ¢, tail- 
structures of the chick are — d'di Pet pointan” Primitive groove ; 
marked out, while those that 
arose during the first day have progressed. Thus, the medullary 
folds close; there is an increase in the number of protoverte- 
bree ; the formation of a tubular heart and the great blood-ves- 
sels; the appearance of the Wolffian duct; the progress of the 
head region; the appearance of the three cerebral vesicles at 
the anterior extremity of the neural canal; the subdivision of 
the first cerebral vesicle into the optic vesicles and the begin- 
nings of the cerebrum; the auditory pit arising in the third 
cerebral vesicle (hind-brain) ; cranial flexure commences; both 
head and tail folds become more distinct; the heart is not only 
