340 OUTLINES OF CHORDATE DEVELOPMENT 



becomes fenestrated and is not completely re-formed until after 

 hatching. At the same time the ventricle is becoming divided 

 into right and left portions by an interventricular septum. 

 This commences as an extension of the spongy wall of the ven- 

 tricle near its posterior apex; it becomes a thick partition, 

 rapidly extends anteriorly, meeting and fusing with the cushion 

 of the auriculo- ventricular aperture, with which the interauricu- 

 lar septum has already connected. Finally the division of 

 the ventricle is completed save for a small antero- ventral aper- 

 ture, the interventricular foramen, by which the root of the 

 dorsal aorta later connects with the left ventricle. 



The bulbus arteriosus too becomes divided before its fusion 

 with the ventricle, by a partition extending from the anterior 

 margin of the pulmonary aortic arch (see below) back to the 

 ventricles. The effect of this is to connect the pulmonary 

 aortic arches with the right ventricle and the systemic aortic 

 arch with the left ventricle. When this bulbus region is ab- 

 sorbed, as described above, the pulmonary arteries and the 

 dorsal aorta arise directly from the right and left ventricles, 

 respectively, and the separate blood streams are established. 



2. The Aortic Arches and the Arterial System 



At the thirtieth hour the heart is connected with the dorsal 

 aorta by a single pair of aortic arches running through the first 

 or mandibular visceral arch. Later an aortic arch forms in each 

 visceral arch, connecting the truncus arteriosus or ventral 

 aorta with the lateral dorsal aortas. The second or hyoid 

 aortic arch forms during the latter part of the second day and at 

 the end of that day the third aortic arch is completed. The 

 fourth is formed by the end of the third day and during the 

 fourth and fifth days fifth and sixth aortic arches are formed in 

 the tissues posterior to the last (fourth) visceral pouch (Fig. 134, 

 .A). Of these arches the fourth and sixth are the best developed, 

 while the fifth is present only as a transitory and incompletely 

 developed vestige. 



This embryonic aortic arch system is converted into the adult 



