500 EMBRYOLOGY OF THE CHICK 



the dorsal end of each mandibular arch and at their anterior edge, a small 

 branch grows downward and forward during the fourth and fifth days. 

 Such branch or branches are called maxillary processes. There is a tri- 

 angular median process growing toward these maxillary processes from 

 the front of the head, known as the fronto-nasal process. The maxillary 

 processes form the upper jaw or the maxillary bones. The maxillary 

 processes do not fuse with each other, but to each side of the fronto- 

 nasal process. When this union does not become complete, the well- 

 known abnormality of hare-lip results. 



The formation of clefts and arches may be understood the better by 

 the following illustration from Professor Reese : 



With the hands in front of the body (the palmar aspect of each 

 hand directed mesiad), and pointed downward, "bring the tips of the 

 fingers together, the fingers of each hand being slightly separated. The 

 thumbs should, at first, be closely pressed against the forefingers, and 

 should be considered as fused with them. If the fingers and hands are 

 slightly bent, there will be a space between the two hands that may be 

 taken to represent the pharynx of the chick, while the four fingers will 

 represent the first four gill arches, and the spaces between the fingers 

 will represent the first three gill clefts. The closure of the visceral clefts 

 may be represented by bringing the fingers of each hand together. The 

 forefingers, which should, in reality, be the only ones which actually 

 meet in the midventral line, will represent the mandibular arch, forming 

 the lower half of the mouth. The formation of the maxillary arch, by 

 processes budded out from the upper ends of the mandibular arch, may 

 be represented by separating the thumbs from the forefingers, and point- 

 ing them toward each other without letting them come in contact; the 

 triangular space between the thumbs, thus held, being fulfilled in the 

 imagination by the fronto-nasal process. The angles between the thumbs 

 and forefingers will represent the angles of the mouth. Of course, to 

 make the comparison more striking, there should be one more finger to 

 represent the hindermost arch and cleft, but as the hinder arches and 

 clefts form no part of the adult chick, this omission is of little impor- 

 tance." 



THE CIRCULATORY SYSTEM (Fig. 297) 



As has been stated, there are already two or three pairs of aortic 

 arches present, by which blood is carried from the bulbus arteriosus 

 around the pharynx to the dorsal aorta. It will be noted that the first 

 aortic arch lies in the first (mandibular gill arch), the second in the hyoid 

 fold, and so on, each bearing a distinct relation to the correspondingly 

 numbered gill-fold. 



The heart, 'which it will be remembered is attached only at the 

 cephalic and caudal ends, is growing rapidly and twisting upon itself. 

 The venous or atrial side is the more stationary. This side, originally, 

 lay caudal to the arterial or conus end of the heart, but in the twisting, 



