48 DEVELOPMENT OF LYMPHATIC SYSTEM, FISHES 



We have already seen that the constituent vessels of which the 

 embryonic ground-plan of the lymphatic system is composed are 

 the same in ganoids as in the trout. It is interesting to note, 

 however, that the order in which these vessels appear in the 

 embryo, in the course of their ontogenetic development, is some- 

 what different in ganoids than in the trout. 



In ganoids (fig. 12) the medial pharyngeal lymphatic develops 

 into a large and prominent sinus (4) which drains the lateral walls 

 of the head (21) and pharynx before a continuous lateral pharyn- 

 geal lymphatic (3'") has been formed. In the trout embryo, on 

 the other hand, the medial pharyngeal lymphatic (4, fig. 10) is 

 one of the last of the principal lymphatics of the pharynx to be 

 developed, and it then serves only in a subsidiary capacity, 

 after connecting with the lateral pharyngeal lymphatic, of aiding 

 the lymph collected by the latter to drain into the veins. In 

 the earlier embryonic stages of ganoids we therefore find that 

 the medial pharyngeal lymphatic plays a more important r61e 

 than the lateral pharyngeal lymphatic in conveying lymph to 

 the venous circulation, while the reverse is the case in the 

 trout. 



The superficial facial lymphatic (21, fig. 12) also makes its ap- 

 pearance in the embryos of ganoids (Amia and Lepidosteus) 

 before a continuous lateral pharyngeal lymphatic has been 

 formed. In the trout embryo, on the other hand, the super- 

 ficial facial lymphatic does not make its appearance until a rela- 

 tively late stage of development, and until after a continuous 

 lateral pharyngeal lymphatic has been formed (21, fig. 11; com- 

 pare with fig. 10). 



In the more advanced stages, in which a continuous system of 

 lymphatics has been established, the medial pharyngeal lymphatic 

 serves in a similar manner, in both the trout and ganoids, as a 

 prominent pathway or sinus, through which the lateral pharyn- 

 geal and superficial facial lymphatics drain into the veins (4, 

 figs. 11 and 13). 



For the reason that the medial pharyngeal lymphatic is a 

 structure of relatively greater importance in the embryos of 

 ganoids than in those of the trout, the earliest stages of its de- 



