32 HENRY MCELDERRY KNOWER 



fluid runs out into the lateral system through the more poste- 

 rior connectives. The result of this is, of course, to obscure 

 the primary importance of the anterior members of the series 

 or may cause it to be overlooked, and this is increasingly true 

 of later stages, the posterior connectives playing a more im- 

 portant part as development proceeds, and contributing to the 

 characteristic enlargement of the lateral system in late stages. 



ESTABLISHMENT OF THE DORSAL LYMPHATIC SYSTEM 



Coincident with the changes in other regions, the dorsal 

 lymphatics form a well-developed system in these early stages. 

 Beginning anteriorly in a plexus which extends back on the 

 cord for several segments over-arched by the dorsal blood 

 vessels (fig. 2), the dorsal tract is continued by a single vessel 

 into the tail. Figures 2, 5, 6 and 7 illustrate the considerable 

 development reached by the dorsal lymphatics in this period. 



The plexuses have no fixed invariable pattern in early 

 stages, while their vessels are becoming adjusted to the altered 

 conditions of the tissues as development progresses from front 

 to back. The arrangement, at first irregular, exhibits a more 

 constant pattern as conditions become stabilized in the dorsal 

 region, the large lymph trunks typical of later stages forming 

 secondary paths through the plexus first organized. Injec- 

 tions may at times follow an exceptional path under the influ- 

 ence of some special resultant of forces exerted at the time 

 of the operation. 



With these considerations in mind, it is possible to interpret 

 injections of the dorsal system in terms of development. 



Most injections of this period, like those in figures 4, 5, 6, 

 etc., demonstrate two or more vessels extending from the 

 margin of the parent lymph plexus to connect with a system 

 of dorsal lymphatics organized to a varying extent in the 

 different specimens. Injections are usually of two types ; one 

 lot furnishing only first stages ; the other a relatively elabo- 

 rate system, establishment of which is a difficult process to 

 follow, on account of the rapidity of distribution of the vessels 

 in this period. 



