EMBRYONIC GROUND PLAN IN GANOIDS 17 



tached in the more advanced embryonic stages, as in the 17.5 

 mm. brook trout (fig. 11), and as early as in an 18.25 mm. rain- 

 bow trout embryo. On treating an 18.25 mm. rainbow trout 

 embryo with chloretone the lateral pharyngeal lymphatic and 

 its anterior continuation in the head region, the superficial facial 

 lymphatic, rapidly filled with blood. A section taken through the 

 caudal end of the subocular lymph sac of this embryo (fig. 16) 

 shows that a partition has been formed between the sac (1) 

 and the lateral pharyngeal lymphatic (3) which now prevents the 

 blood from passing from the latter into the sac. A section (fig. 

 17) taken through the same embryo, somewhat anterior to the 

 level of the former section, shows the independent character of 

 the blood-filled superficial facial lymphatic (21), as well as its 

 relation in the head region to the subocular lymph sac (1). 



V. THE EMBRYONIC GROUND-PLAN OF THE LYMPHATIC SYSTEM 

 IN GANOIDS AND THE SUBSEQUENT TRANSFOR- 

 MATIONS WHICH IT UNDERGOES 



The embryonic ground-plan of the lymphatic system is exactly 

 the same in ganoids as that described above for the twenty-two- 

 day rainbow trout (fig. 10). In Amia and Lepidosteus, as in the 

 trout, the subocular lymph sacs, the lateral and medial pharyn- 

 geal lymphatics, together with the precardinal lymphatics, con- 

 stitute the framework upon which the structure found in the 

 later stages of the embryo and in the adult is necessarily built. 

 I have paid more attention to Amia than to Lepidosteus and 

 will therefore confine my remarks, for the most part, to the con- 

 ditions observed in this form. 



In Amia and Lepidosteus each subocular lymph sac (1) early 

 establishes a direct communication with the precardinal vein in 

 the immediate neighborhood of .the sac (29, fig. 12), instead of 

 with the lateral pharyngeal lymphatic as is the case in the trout 

 (fig. 10). In embryos of Amia measuring somewhere between 

 10.5 and 12.5 mm. in length and in those of Lepidosteus soon 

 after they have attained the 14 mm. stage, the subocular lymph 

 sacs (1) then become detached from the veins (fig. 13). From 

 this time on, with the exception of a doubtful communication in 



