306 GEO. S. HUNTINGTON 



tions with independently developed peripheral lymphatic chan- 

 nels, joins the jugular lymphsacs and through them attains its 

 entry into the venous system. 



■ From its earliest inception the reptilian periaortic sinus is at 

 no point in relation to the venous system. It is intimately re- 

 lated to the dorsal aortae, but there are, at the site of its devel- 

 opment, no large embryonal venous channels corresponding to 

 the mammalian azygos (post- and supracardinal) trunks. Con- 

 sequently the developing thoracic, or rather coelomic lymphatic 

 sinuses of the reptile never come into intimate genetic or topo- 

 graphical relations with axial veins. 



Further, the axial periaortic mesenchyme of the reptilian em- 

 bryo is not the site of an active haemopoesis. Consequently, in 

 strong contrast with the avian type, the reptilian homologues 

 of the thoracic ducts never become haemophoric. 



The reptile offers hence in this region the clearest and least 

 complicated illustration of the basic principle underlying all 

 vertebrate angeiogenesis in general and all lymphatic ontogeny 

 in particular, viz., the formation of a system of connected chan- 

 nels, developed by fusion of originally separate and independent 

 intercellular mesenchymal spaces, not complicated by any rela- 

 tion whatsoever to the systemic veins (fig. 18, B), nor charged 

 with the haemophoric function of conveying red blood-cells devel- 

 oped in situ into the general haemal circulation. The bird (tig. 

 19) follows the general reptilian type of development with the 

 following important modifications: 



The periaortal mesenchyme of the chick is the site of a most 

 active and abundant intraembryonic haemopoesis in situ. Masses 

 of developing blood cells differentiate as axial strands [the 'mesen- 

 chymal cords' of Sala (24)] around the aorta, directly from the 

 indifferent periaortic mesenchymal syncytium (fig. 19, A, 2). 

 Subsequently the anlages of the thoracic ducts appear in this 

 periaortic area as isolated intercellular mesenchymal clefts and 

 spaces (fig. 19, B, 3). These spaces become confluent, receive 

 the blood cells developed in the mesenchymal blood-islands, and 

 convey them through the channels of the thoracic ducts to the 

 jugular lymphsacs, and through them into the circulating venous 



