THU ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 553 



So far as regards the purpose of the umbilical circulation, it 

 subserves in Reptiles and Birds the function of respiration. For the 

 allantois, when it has become larger, in the Chick for example, 

 applies itself closely to the serosa and spreads itself out in the 

 vicinity of the air-chamber and underneath the shell, so that the 

 blood circulating in it can enter into an exchange of gases with the 

 atmospheric air. It loses its importance for respiration in the egg 

 only at the moment when the Chick with its beak breaks through 

 the surrounding embryonic membranes, and breathes directly the air 

 contained in the air-chamber. For the conditions of the circulation 

 are now altered throughout the whole body, since with the begin- 

 ning of the process of respiration the lungs are in a condition to take 

 up a greater quantity of blood, resulting in a degeneration of the 

 umbilical vessels (compare also p. 584). 



The umbilical or placental circulation in Mammals (fig. 139 A I) 

 plays a still more important role ; for here the two umbilical 

 arteries convey the blood to the placenta. After the blood has 

 been laden in this organ with oxygen and nutritive substances, it 

 flows back again to the heart, at first through two, afterwards 

 through a single umbilical vein (p. 584). 



B. The further Development of the Vascular System up to the 

 Mature Condition. 



(a) The Metamoi'phosis of the Tubular Heart into a Heart 

 with Chambers. 



As has been shown in a preceding section, the heart of a Verte- 

 brate originally has for a short time the form of a straight sac, which 

 sends off" at its anterior end the two primitive aortic arches, while it 

 receives at its posterior end the two omphalomesenteric veins. The 

 sac lies far forward immediately behind the head on the ventral side 

 of the neck (fig. 304 h), in a prolongation of the body-cavity (the 

 parietal or cervical cavity). It is here attached by means of a 

 mesentery of only brief duration, which stretches from the alimentary 

 canal to the ventral w r all of the throat, and which is divided by 

 the cardiac sac itself into an upper [dorsal] and an under part, or 

 mesocardium posterius and anterius. 



During the first period of embryonic development the heart is 

 distinguished by a very considerable growth, especially in the longi- 

 tudinal direction ; consequently it soon ceases to find the necessary 



