FOKMATION OF THE VASCULAR SYSTEM. 747 



that the third and fourth receive the greater part of the blood, while 

 in the meantime a fifth arch makes its appearance behind the fourth ; so 

 that in this way there are still four permeable arches. 



(2125.) While these changes are going on in the vascular canals, the 

 first branchial fissure gradually closes ; and to make up for this, a new 

 one is formed between the arch which originally was the fourth and that 

 last developed. 



(2126.) At the commencement of the fifth day there are consequently 

 again four vascular arches and three branchial fissures on each side ; but 

 not the same as those of the third day, since one arch and one fissure 

 have disappeared, and have been replaced by similar parts. During the 

 fifth day, the vascular arch which at first was the second is obliterated, 

 and the two succeeding ones become increased in size ; but at the end of 

 the fifth day all the branchial fissures are effaced, being filled up with 

 cellular tissue, and no trace of them is left. The remainder of the 

 metamorphosis seems to depend principally upon changes that occur in 

 the bulbus arteriosus (fig. 373, >), which is by degrees converted into the 

 bulb of the aorta. This part of the arterial system, from being a single 

 cavity, about the fifth day divides into two canals, which become gradu- 

 ally more and more separated and bent upon themselves. The separa- 

 tion of the bulbits arteriosus into two vessels is, in the opinion of Pro- 

 fessor Baer, owing to the circumstance that the ventricles gradually 

 become separated by a septum, which, as it grows more complete, 

 causes two distinct currents of blood to be propelled from the heart. The 

 current coming from the right ventricle arrives sooner than the other at 

 the vascular arches, and rushes through the two posterior and through 

 the middle arch of the left side, while the gush of blood from the left 

 ventricle fills the two anterior arches and the middle arch of the right 

 side, a circumstance depending on the course impressed upon the cur- 

 rents derived from the two ventricles. Each current becomes more and 

 more distinct ; and at last each is provided with a proper channel, form- 

 ing the trunks of the future pulmonary artery and of the future aorta. 



(2127.) It will be seen that as yet the real aorta does not exist ; for 

 at this period of the metamorphosis all the blood passes through the 

 vascular arches that remain into the dorsal vessel (fig. 374, m), which is 

 formed in the same manner as the aorta of Fishes, by the union of the 

 branchial vessels. 



(2128.) While the branchial fissures penetrated into the pharyngeal 

 cavity, the branchial vessels were contained in the corresponding bran- 

 chial arches ; but as soon as these fissures disappear, the vascular trunks 

 abandon the neighbourhood of the pharynx and begin to assume the 

 character they afterwards present. 



(2129.) The most posterior arch of the left side gradually disappears, 

 and on the seventh day of incubation is no longer recognizable ; whilst 

 in the meantime the current of blood from the right ventricle is directed 



