72 EARLY EMBRYOLOGY OF THE CHICK 



through of the dorsal and ventral mesocardia is of interest 

 aside from the fact that it leaves the heart free to undergo 

 changes in shape. It makes the right and left ccelomic cham- 

 bers confluent, the pericardial region thus being the first part 

 of the ccelom to acquire the unpaired condition characteristic 

 of the adult. 



Although there are as yet no sharply bounded subdivisions of 

 the heart, it is convenient to distinguish four regions which later 

 become clearly marked off from each other (Fig. 23). The 

 most caudal part of the heart where the omphalomesenteric 

 veins join is the sinus venosus; the caudal part of the region 

 of the heait which is dilated and bent to the right will become 

 the atrium ; the cephalic part of the heart bend is the ventricular 

 region; and the region where the ventricle swings into the mid- 

 line and becomes narrowed is known as the bulbo-conus ar- 

 teriosus. Approximately at the stage of development indicated 

 in Figure 23 irregular twitchings occur in the heart walls, but 

 regular pulsations are not established until about the 44th hour 

 of incubation. 



The Formation of the Intra-embryonic Blood Vessels. Co- 

 incident with the establishment of the heart, blood vessels have 

 arisen within the body of the embryo. Concerning the exact 

 nature of the process of blood vessel formation there is some 

 disagreement. The weight of evidence seems to indicate 

 that the early vessels are formed from mesodermal cells which 

 lie in the path of their development. They grow by organi- 

 zation of cells in situ as a drain might be built from bricks 

 already deposited along its projected course. In later stages 

 it seems probable that vessels extend by the formation of bud- 

 like outgrowths from their walls, as well as by organization of 

 cells *in their path of development. When first formed, the 

 blood vessel walls are but a single cell in thickness. There 

 is no structural differentiation between arteries and veins 

 until a considerably later period. Recognition of the vessels 

 depends wholly, therefore, on determining their course and 

 relationships. 



The large vessels connecting with the heart are the first of 

 the intra-embryonic channels established. From the bulbo- 

 conus arteriosus the paired ventral aortic roots extend cephalad 

 ventral to the fore-gut (Fig. 23). At the cephalic end of the 



