126 THE DEVELOPMENT OF THE CHICK 
stage (Figs. 63 and 65) to a point a little behind the last meso- 
blastic somite; beyond this the medullary folds diverge and are 
lost to view towards the hind end of the primitive streak. We 
may distinguish a cephalic portion (brain or encephalon) and a 
trunk portion (spinal cord or myelon) of the neural tube; the 
boundary lies between the fourth and fifth somites, for the first 
four somites enter into the composition of the head. The brain 
is thus at this time about as long as the portion of the cord formed 
or indicated by the medullary folds. For description, see p. 108. 
Alimentary Canal. The alimentary canal and its appendages 
exist only potentially in this embryo in the form of the splanchno- 
pleure, except in the head. The cephalic enteron of this stage 
corresponds to a large part of the pharynx. The oral plate has 
already been described in connection with the sagittal section 
(Fig. 67). In transverse section the extreme anterior end of the 
fore-gut is quite narrow, elsewhere it is very wide laterally, and 
in one place its lateral expansions come in contact with the 
ectoderm on each side and fuse to it, thus indicating the hyoman- 
dibular cleft. The floor and lateral walls of the pharynx are com- 
posed of columnar cells, the roof of flattened squamous cells 
(Fig. 54). 
Vascular System. The heart lies in the parietal cavity be- 
neath the pharynx; it is bent near its middle to the right. It is 
an undivided double-walled tube, the internal wall or endocardium 
being a continuation of the blood-vessels, and the external wall, 
myocardium or muscular heart, being a duplication of the wall 
of the pericardium. It has not yet reached the stage of regular 
contraction, though it may be observed to twitch from time to 
time. The chambers of the heart are indicated, but not clearly 
defined at this time; one can only say that the posterior end is 
the venous end from which the sinus and auricles are to form, 
and the anterior two thirds, the arterial end, destined to form the 
ventricles and bulbus. 
The endocardium is continued anteriorly into the ventral 
aorta, which divides on each side of the oral plate (Fig. 64), to 
form the mandibular arches that describe a loop around the 
anterior end of the fore-gut and are continued posteriorly as 
the dorsal aorte, which run above the roof of the pharynx, lateral 
to the notochord, into the trunk, where they lie ventral to the 
nephrotome, and send off short blind branches (segmental arteries) 
