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- 

 dihular 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 aortce, 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) 



