54 DEVELOPMENT OF LYMPHATIC SYSTEM, FISHES 



If we analyze these observations more closely, we find that up 

 to the eighteenth day the anlagen of the- lateral pharyngeal 

 lymphatic cannot, in all cases, be uniformly injected from the 

 veins at the typical points of lymphatico-venous entry. From 

 the eighteenth day on, however, with one exception on the 

 twentieth day, no difficulty was experienced in forcing the in- 

 jecta into the anlagen of the lateral pharyngeal lymphatic from 

 the veins. The study of sections of injected embryos has shown 

 that the inability to inject the anlagen of the lateral pharyngeal 

 lymphatic in the earlier stages is due to the circumstance that, 

 in some cases, they do not communicate with the veins, or, if a 

 communication is present, it may be too narrow to permit the 

 passage of the injecta into the anlagen from the veins. 



On the eighteenth day we meet with the nearest approach to a 

 uniformity, in which the lateral pharyngeal lymphatic has at- 

 tained a stage of development which might be regarded as typical 

 for this particular day. At this stage (in 18 of the 21 embryos 

 injected) the lateral pharyngeal lymphatic was represented by a 

 continuous vessel on both sides of the embryo which extended 

 between the cardino-Cuvierian junction and the caudal end of the 

 otocyst, at which point, in some cases, it communicated with the 

 veins. This same condition was also met with to a lesser de- 

 gree on the sixteenth (in 3 of the 16 embryos injected) and seven- 

 teenth days (in 5 of the 11 embryos injected) and is illustrated by 

 figure 6, which represents a reconstruction of a seventeen-day 

 steelhead trout embryo. This condition signifies, I believe, the 

 attainment of a necessary preliminary stage of development in 

 which the establishment of the lymphatic drainage system for the 

 posterior portion of the pharynx may be regarded as complete. 

 This drainage system for the posterior region of the pharynx is 

 still independent, however, of that of the subocular lymph sac. 



From the eighteenth to the twenty-second day no indication 

 of a uniform condition was met with and, during this period, 

 the further development of the lymphatic system is chiefly con- 

 cerned in establishing a communication between this posterior 

 drainage system of the pharynx and the subocular lymph sac. 

 A study of sections of these injected trout embryos has shown 



