The Origin and Development of the Lymphatic System. 49 



group under " Early Venous Stages," is, I think, a reconstruction of 

 the cardinal veins before the lymphatics begin (and is therefore to 

 be compared with Evans' (34) fig. 3, and with my fig. 4 in this paper). 

 In their fig. 22 the fourth segmental vessel is shown and described 

 (page 226) as " formed by the confluence of two extended para-neural 

 channels." This is intended to indicate the beginning of the lym- 

 phatics, but since their fig. 43 from an embryo 10 mm. long shows 

 undoubted lymphatics in the blood-packed stage, I think it is prac- 

 tically certain that the specimen measuring 5 mm: does not show lym- 

 phatics. It is not necessary to prove that veins do not open into the 

 tissue spaces around nerves. 



The description of the stages between 5 and 10 mm. is bound up 

 with the discussion of "fenestration." In reconstructing the veins 

 Huntington and McC'lure have noted the pattern of the developing 

 veins, which is especially well shown in fig. 4, in connection with the 

 pericardial veins. In this specimen the pericardial veins drain into a 

 loop which connects the anterior cardinal vein with the duct of Cuvier. 

 The formation of this loop is a part of the process by which the peri- 

 cardial veins shift from the duct of Cuvier to the anterior cardinal 

 vein. Numerous small loops are seen along the anterior cardinal 

 veins in figs. 4 and 5. They are characteristic of the veins. They 

 suggested to Huntington and McClure a vague conception of a rela- 

 tion to the lymphatics which they expressed as follows (p. 204 in 54) : 

 " In a subsequent stage confluence of these ' f enestras ' results in the 

 more or less complete separation of the primary vein into two parallel 

 secondary channels, which appears to be a uniform principle in the 

 development of parallel venous trunks. Or, as in the case of the jugu- 

 lar lymph sac, further extension of the same process may result in 

 separating from the main venous channels elements which unite to 

 form a closed sac entirely distinct from the vein from which it arose. 

 The term ' fenestration ' means, therefore, in the sense in which it is em- 

 ployed in this paper, one of the last stages in the definite crystalli- 

 zation of the venous system out of an indefinite antecedent plexiform 

 condition, and the determination of an important element in lymphatic 

 organization, closely associated with the embryonic venous system." 

 The lymphatics which they thought formed by this process they termed 

 " veno-lymphatics." Both terms " fenestration " and " veno-lymphat- 

 ics " should drop out; there are no veno-lymphatics, for the lymphatics 

 bud directly from the veins as lymphatics, and fenestration is a term 



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