46 DEVELOPMENT OF LYMPHATIC SYSTEM, FISHES 



precardinal vein (6), with the anlage (5") near the otic lymph 

 sac, nor is it connected with the vein by a solid endothelial 

 strand. In the twenty-two-day rainbow trout (fig. 10) this 

 anlage (5'") connects with the precardinal sac (5") which arises 

 near the otic communication and with the former forms a con- 

 tinuous vessel or sac (5) which can now be injected from the veins. 



The only point at which an extraintimal replacement of a 

 degenerating vein by a developing lymphatic could occur in the 

 trout would be in connection with the precardinal lymphatics 

 although, as far as I have been able to determine, such a replace- 

 ment does not take place. It is evident, however, if the pre- 

 cardinal vein (6) in figure 39 were to undergo atrophy and the 

 precardinal lymphatic (5) to increase in size, we would then have 

 a typical example of the extraintimal replacement of a vein by a 

 lymphatic, as described by Huntington and the writer in con- 

 nection with the development of the thoracic ducts and mesen- 

 teric lymphatics in mammals. 



In addition to the posterior anlagen of the precardinal lym- 

 phatics just mentioned, additional independent anlagen arise in 

 the region anterior to the otic lymph sacs. These anlagen (5') 

 make their appearance on about the twenty-first day in the 

 rainbow trout and, as shown in the reconstruction of the twenty- 

 two-day rainbow trout embryo (fig. 10) they lie remote from the 

 point at which they will subsequently connect with the lateral 

 pharyngeal Ij-mphatic (7). These anlagen (5') are also shown in 

 section (fig. 33) in which it is seen that they arise contiguous to 

 the precardinal vein (6). 



The question concerning the method by means of which con- 

 nections are established between the independent anlagen of 

 the lateral pharyngeal lymphatic, and between the lymphatics 

 and the veins, is of secondary consideration at present and will 

 be fully considered further on (Section XIII, p. 57). The main 

 question at issue is whether or not such independent endothelial- 

 lined anlagen actually exist. If they do exist, the contention 

 that lymphatic endothelium is specific, in the sense that it is 

 invariably derived from that of the veins, can no longer be 



