24 HENRY MCELDEREY KNOWER 



Again it must be repeated for this period that I have found 

 no rudiments which can be called * lymphatic' associated with 

 the submaxillary blood vessels. 



But though no lymphatic rudiments are found in ventral 

 regions in these early stages, the dorsal systems of stage 2, 

 as compared with stage 1, exhibit a period of rapid develop- 

 ment in certain groups of vessels already identified as related 

 to the first lymphatics. 



In passing from the stage of figure 1 to that shown in figure 

 2, the earlier vascular conditions in the dorsal systems are 

 not greatly altered. There is steady extension of plexuses 

 and of connecting units, with enlargement and better defini- 

 tion of vessels everywhere. As the body segments and organs 

 expand, the blood vessels become spread apart, as already 

 noted above for the tail plexuses (p. 14) comparing these 

 stages. The dorsal and lateral connecting loops between the 

 intersegmental veins are increased and reenforce the lateral 

 veins (posterior vertebral) which are now quite conspicuous. 



Distribution of lymphatics in second period 



The most pronounced changes in both lymphatics and veins 

 are found in the region of the anterior segments dorsal to the 

 pronephros, where the first lymphatics appeared earlier, 

 attached to the third intersegmental vein. The early ' lymph 

 heart plexus' (L.Ht.Plx.) at this time is illustrated in figure 2, 

 where its close-meshed network is seen dorsal to the heart, 

 as before, but now occupying a more extended area super- 

 ficial to the first four anterior segments. A connection with 

 the dorsal lymphatics can be traced from the periphery of 

 the plexus dorsally over the roof of the hind-brain. Such 

 double injections demonstrate conclusively the continued pres- 

 ence of the primary segmental veins of the region deep to the 

 lymph plexus at this stage, and give the general nature of 

 the interrelations of the two systems, with independence of 

 the two already conspicuous. However, special injections of 

 the region must be looked to for particular details. 



