14 



teristics of the special vessels which give rise to the first 

 lymphatics will be understandable. 



Such a preliminary review will naturally start with the 

 dorsal capillary beds, which are immediately related to the 

 lymphatics in their first stages. Hence the neural blood ves- 

 sel plexuses of the spinal cord and brain come first, and are 

 found along the entire ventral surface of the neural tube as a 

 fine network of small vessels in which two longitudinal vessels, 

 'the ventral spinal arteries,' are differentiated. The net 

 spreads upward at the sides, part way dorsally (fig. 1), but 

 no blood vessels are present on the dorsal surface of the cord 

 or hind-brain at this time. 



The arteries supplying the cord plexus are primary seg- 

 mental dorsal branches of the aorta. They are seen (Dsl.A.) 

 in both figure 1 and figure 2, of the next stage, deep to the 

 lateral veins running directly dorsalward to the under surface 

 of the cord along the mid-ventral line. (The figures are best 

 observed through a hand lens, which emphasizes depth 

 relations.) 



In the region of the tail in the earlier stage (fig. 1) the 

 segmental arteries follow one another in close sequence, to 

 become more widely spaced later, with the lengthening of the 

 tail in older larvae (fig. 2). 



In both stages illustrated in figures 1 and 2 the dorsal fin 

 plexus, formed by loops from the neural plexus, is still quite 

 undeveloped. In the earlier stage (fig. 1), the small close 

 meshed vessels and short outrunning branches are evidently 

 just beginning the invasion of the inner part of the tail fin 

 (fig. 1). In figure 2, for the next stage, though the meshes 

 are wider and the vessels more defined, the outrunning 

 branches are still merely incipient vessels. A similar fin plexus 

 is formed in the ventral tail fin. These fin plexuses, which 

 have been studied by Clark ('18) so effectively in later stages 

 when visible in the living transparent tail, have, as yet, no 

 circulation at this early period, and cannot be seen without 

 injection. They are evidently merely starting their extension 

 and development. 



