20 HENRY MCELDERRY KNOWER 



the dorsolateral systems of the first stage; and it may be 

 stated further that injections indicate that none are found so 

 early in the ventral systems. Since, however, claims have 

 been made that primary lymphatic rudiments are found in 

 ventral regions of toad embryos, the ventral systems must be 

 examined in detail in frog embryos. 



Ventral vessels 



The ventral zone of organs is supplied by a series of arteries 

 from the aorta, which, posteriorly, form the plexus of the 

 ventral fin in the tail and in the trunk break up into a network 

 along the entire surface of the yolk-filled gut. In early stages 

 this vitelline plexus drains ventrally on either side into a 

 strong vein, the two vitelline veins thus formed running for- 

 ward into the heart on each side of the liver rudiment. In 

 passing the liver, connections are made with hepatic veins. 

 This arrangement marks another primitive feature of the 

 young embryos, later larva having only a single vitelline vein 

 on the right. Compare figures 1 and la with figures 2, 2a and 

 2b for these relations. 



Figure 1 also shows the oesophageal plexus running down 

 from lateroventral arteries given off at the anterior end of 

 the aorta, just inward from the pronephric sinus. As already 

 explained, these arteries form the glomerulus, from which 

 efferent vessels continue ventrally on the oesophagus. 



Omitting special consideration of the aortic arches, clearly 

 seen lying deep in the walls of the pharynx, the more super- 

 ficial submaxillary plexus must be thoroughly examined, since 

 the origin of the first lymphatic rudiments of the toad from 

 veins of this plexus or from tissues adjacent has been de- 

 scribed far apart from the anterior lymph hearts and entirely 

 unconnected with them. 



Figure la, a ventral view of the same specimen of R. pal. 

 from which figure 1 was drawn, gives a picture of the sub- 

 maxillary vessels injected in an early stage of the plexus. 

 The vascular conditions exhibited appear to be at about the 

 same stage of development as that in Kampmeier's figure 29 



