30 DEVELOPMENT OF LYMPHATIC SYSTEM, FISHES 



The fact demonstrated in the above experiments that connective 

 tissue-cells are changed in form by physical agents into flat, closely 

 disposed cells, the outline of which may be defined by silver salts, 

 makes tenable the conclusion that the exposed connective tissue cells, 

 exposed through sacrifice of surface mesothelial cells of pleura, peri- 

 toneum, or pericardium or of the lining endothelial cells of vessels, 

 may become flattened by pressure or friction or both, resulting in 

 regeneration of the surface cells. 



Up to this point we have considered only the earliest stages in 

 the development of the subocular lymph sacs and will now turn 

 our attention to the later stages. 



Soon after their first appearance as definite single-chambered 

 structures, the subocular l^^mph sacs of the trout gradually in- 

 crease in size and extent, and by the time a connection is about 

 to be established between them and the lateral pharyngeal lym- 

 phatic, they have become relatively huge structures (fig. 42). 



As shown by injection experiments and by reconstructions 

 (figs. 1 to 10, inclusive), the subocular lymph sacs do not increase 

 in extent; in a caudal direction. The base or caudal end of the 

 sac remains practically stationary during all stages of develop- 

 ment and lies just in front of the hyoidean artery (15).^ The 

 extension of the subocular sacs takes place in an anterior direc- 

 tion and can be followed through the different stages of develop- 

 ment. During the course of their forward extension the sub- 

 ocular sacs gradually approach and finally come into close 

 contact with the infraorbital veins, and it might be reasonably 

 expected that a temporary communication, as occurs in ganoids, 

 would be established between the independently formed sacs and 

 the veins. Such, however, has not been found to be the case. 

 In no instance has a communication been observed between the 

 subocular lymph sacs of the trout and the infraorbital veins, in 

 sections either of injected or uninjected embryos. Even if a 

 communication were established it would not be a primary one 



* In the trout (fig. 10) and in Amia (fig. 12) the first or most anterior pair of 

 efferent aortic arches {15) subsequently become detached from the roots of the 

 doi'sal aorta and then function as the efferent (ophthalmic) arteries of the pseudo- 

 branch {15', figs. 11 and 13). For a detailed description of the pseudobranchial 

 arteries in adult teleosts see Silvester's paper ('04) The blood vascular system of 

 the tile-fish (Lopholatilus chamaeleonticeps) . 



