ORIGIN OF VASCULAR TISSUES 119 



the otocj^st. See also Reagan and Thorington (55), figure 15. 

 The 'spUtting' which Stockard often found in the anterior por- 

 tion of the 'stem vein' probably represents precardinal veins. 

 The 'cardinal vein' of Stockard's figure 9 (65) is almost certainly 

 a duct of Cuvier with its anterior continuation, the precardinal. 



At the thirtj^-second session of the American Association of 

 Anatomists, the writer presented evidence that erythrocytes may 

 develop locally in many regions in which Stockard claims they 

 never form. The objection was immediately raised that the 

 red blood cells observed in these positions had migrated. Such 

 objections had been anticipated (53, p. 116) and an'&wered as 

 follows: ''I am not at present concerned with the possibility of 

 those cells having migrated from some other position; the pres- 

 ent situation is relieved of such perplexing considerations when 

 Stockard goes so far as to say that the heart, anterior vessels, 

 anterior yolk, and liver are regions in which wandering blood an- 

 lagen never make themselves manifest. He also speaks of eryth- 

 rocytes 'originating' on the yolk when it is certain that their ul- 

 timate anlagen did not occupy that position." 



Thus it is seen that blood cells are described as 'originating' 

 (64, p. 125) on the yolk, although their anlagen migrated there 

 from some other position. Have we not the same privilege even 

 if it should be demonstrated that cell-migration is responsible 

 for anterior blood cells? Is it reasonable to suppose that eryth- 

 rocytes should migrate, when in chemically treated embryos 

 which are perfectly normal "so far as the blood anlage is con- 

 cerned" the leucocytes and lymphocytes of Stockard are unable 

 to move from the head regions? The differentiation of a mesen- 

 chymal derivative on the teleost yolk-sac in the absence of cir- 

 culation necessitates migration owing to the original absence of 

 mesench\ane on the yolk. On the other hand, the mesenchyme 

 is so widely distributed throughout the embryo's body that it 

 is not necessary to assume that a blood-producing mesenchyme 

 cell had migrated to a given place within the embryo's body 

 from some other region. Mesenchjaiie cells undoubtedly can 

 migrate, but the process of migration in case of intraembryonic 

 hematopoesis would scarcely suggest itself except in support of 



