122 JOHN LEWIS BREMER 



plexus, while the lower curve includes conus arteriosus and lateral 

 heart. The upper curve corresponds with the pharynx, the lower 

 is below the pharynx and associated with the coelom. 



If we notice the position of the lateral heart in figs. 4 to 7 we 

 can see it is gradually rolled over and inverted, at its anterior end, 

 so that whereas in fig. 4 it projected dorsally into the coelom and 

 therefore was connected with the other vessels ventrally only, 

 in fig. 6 the projection is distinctly lateral. In fig. 7, from a rab- 

 bit of eleven segments, the lagging lateral hearts have ultimately 

 met in the median line and partly fused, but not until the pharynx 

 floor has been completed and the ventral aortic network established, 

 so that the inverted and fused lateral hearts lie at a distinctly 

 lower level, connected with the ventral aortae by the ascending 

 conus arteriosus, which itself is composed of two fused halves. 

 The blood-vessels of this part of the embryo now lie in three 

 tiers or levels; dorsally, the dorsal aortae, ventrally the heart, 

 and between these two the ventral aortic plexus, joined to the 

 former by the first arch and to the latter by the conus arteriosus. 



On the recognition of this middle tier, the plexus of the ventral 

 aortae, depends the proper understanding of the development 

 of the rest of the aortic arches and of the pulmonary arteries. 

 If we turn back to fig. 3, we shall see that the three levels are 

 already indicated in the angioblast net ; the dorsal aorta, in plexus 

 form, occupies a distinct region toward the median line, the lateral 

 heart lies toward the right of the figure, while, connected with this 

 by the conus arteriosus and with the dorsal aorta by the first 

 arch, a portion of the net remains, expanded longitudinally, and 

 lying beneath the mesial border of the coelom. This is to form 

 the ventral aorta, and is to lie in the floor of the pharynx, while 

 the conus is to lead from this level to the heart, which occupies a 

 more ventral position. 



It will be noticed that in fig. 3 there is an extension of the ven- 

 tral aorta caudad from the conus arteriosus. This may be pre- 

 cocious in this case, for in the two embryos of eight segments, 

 figs. 5 and 6, no such extensions were found, while in the embryo 

 of eleven segments it is again present. This caudad growth of the 

 ventral aortic net lies in the thin sheet of mesoderm between the 



