20 DEVELOPMENT OF LYMPHATIC SYSTEM, FISHES 



I have no definite explanation to offer concerning the signifi- 

 cance of the detachment which the subocular lymph sacs subse- 

 quently undergo from the veins in ganoids, and from the lateral 

 pharyngeal lymphatic in the trout. One might possibly class 

 these sacs as evanescent structures of the same general character 

 as the posterior lymph hearts found in the embryos of certain 

 birds. The condi^ns observed in ganoids, for 'example, are 

 remarkably like those in birds. In the common fowl the posterior 

 lymph hearts function only temporarily in the embryo as struc- 

 tures which communicate with the veins, and, although traces 

 of these hearts can be observed in young chicks thirty-five days 

 after hatching (Sala '00), they never enter into the formation 

 of the lymphatic system in the adult. 



Although I have not made an exhaustive study of teleosts other 

 than the trout, I have reason to suppose that in some of the more 

 highly specialized tefeosts it is quite possible that the subocular 

 lymph sacs, although present in the embryo, may never at any 

 time establish a communication with the veins or with any other 

 lymphatic. In such cases one would expect to find the superficial 

 facial lymphatic to be developed, and to function in the capacity 

 of draining the head region at a relatively much earlier stage of 

 development than is the case in the ganoids and the trout. 



VI. THE DEVELOPMENT OF THE SUBOCULAR LYMPH SACS IN THE 



TROUT 



The subocular lymph sacs can be observed in the living trout 

 embryo during the earlier as well as later stages of their develop- 

 ment; for this reason, as well as on account of the relatively large 

 size they attain, they lend themselves in an unequalled manner 

 to injection experiments in the living form. 



In the living trout embryo the subocular lymph sacs appear as 

 non-pulsatile, translucent, triangular-shaped sacs filled with 

 lymph which can easily be withdrawn from the sacs by means 

 of a cannula. These sacs (1) lie on each side of the head, ventro- 

 medial to the eye (fig. 18), and are lined by an endothelium 

 around which no muscular coat is formed. 



