THORACIC DUCT IN THE CHICK 153 



This is seen near the cephalic end of the large ventral lymph 

 plexus {17) but has no connection with the latter. 



At this point it may be well to state that in the chicks of the 

 Columbia Collection I have found no evidence of extra-intimal or 

 perivenous origin of the lymphatics that make up the thoracic 

 duct, such as described by Huntington in the cat and Kampmeier 

 in the pig. This mode of development appears to be a strictly 

 mammalian specialization. In fact Huntington (3), speaking of 

 the extra-intimal replacement of veins by developing lymphatics, 

 states (p. 155) : 



The association of these (Ij^mphatic) channels, in the mammalian 

 embryo, with certain embryonal venous lines is purely a secondarj^ 

 mechanical and topographical relationship, expressed by the condensed 

 term of 'extra-intimal' development of mammalian systemic lymphatic 

 vessels, and absolutely devoid of genetic significance. This is, without 

 reference to other vertebrate classes, proved by the development within 

 restricted areas in the mammalian embryo of systemic lymphatic chan- 

 nels through the direct confluence of intercellular mesenchymal clefts, 

 not related topographically or in any other sense to the embryonal 

 veins. It is true that in the mammal this independent lymphatic gene- 

 sis is extremely limited, and that the majority of the lymphatic vessels 

 develop in close association with embryonal veins, as products of the 

 confluence of perivenous extra-intimal spaces. But this is merely, as 

 shown by comparison with other amniote embryos, the expression of the 

 peculiar relations obtaining in the mammal between the venous and 

 lymphatic circuits of the vascular system, developed independently of 

 each other. 



Further, Huntington (2), describing the independently formed 

 system lymphatic channels of the reptilian embryo, by confluence 

 of intercellular mesenchymal spaces, states concerning the latter: 



They are not complicated by close topographical relations to adjacent 

 temporary embryonic venous plexuses, as in the mammal, but develop 

 independently by themselves in mesenchymal territory not occupied by 

 hemal vascular elements In both lacertilian and chelonian em- 

 bryos the greater part of the enormously enlarged systemic lymphatic 

 channels develop without any reference whatever to embryonic veins, in 

 mesenchymal areas where the latter are extremely scanty or entirely 

 wanting (pp. 271-273). 



Stromsten (16) has reached similar conclusions in studying the 

 development of the prevertebral peri-aortic sinuses in chelonian 

 embryos. 



