26 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



considered superficially at this time. Yet, in some regions, 

 very suggestive pictures are obtained. Thus in section 4 of 

 slide viii of series 111 (figs. 1, 2 and 3) the distinct appearance 

 of a communication between the coelom cavity proper (78} and 

 the early mesenchymal spaces (77) above described is given by a 

 clearly limited and well defined funnel-shaped stoma, occuping 

 the dorsal extremity of the coelomic cleft (79 in fig. 2), and 

 apparently opening directly into the spaces of the early lym- 

 phatic plexus. The remaining sections of this series figured 

 (figs. 4 to 7) confirm this impression. 



The conditions here described for early embryos of the cat 

 strongly support the views expressed by Marcus 9 in his studies 

 on the lymphatic development of Hypogeophis. The micropho- 

 tographs here given should be compared with his description on 

 pp. 599-601 of the paper quoted, with his text fig. 6, and with his 

 figs. 5 and 6 of plate xvi. 



The early mesodermal spaces here described and figured are 

 lymphatic in character and form part of the extensive temporary 

 network of lymphatic channels which appears for a time during 

 mammalian ontogenesis and which bears a close resemblance to 

 the corresponding lymphatic organization in amphibia and 

 reptiles. The peri-aortic lymphatic sinuses and the exaggerated 

 subcutaneous lymph channels of the earlier mammalian stages are 

 portions of this evanescent and reminiscent system, which subse- 

 quently in large part retrogrades, and either disappears altogether 

 or is extremely modified to meet the definite permanent condi- 

 tions of mammalian lymphatic organization. Thus the early 

 periaortic spaces become much reduced in course of further devel- 

 opment. They then become associated, in a way presently to 

 be described in detail, with elements of the axial venous plexuses 

 of the mammalian embryo and form the anlages of the main 

 segments of the thoracic ducts. This ontogenetic temporary 

 recall of antecedent phylogenetic types of vascular development 

 appears to be chiefly centered, in the mammalian embryo, in 

 the region around the omphalo-mesenteric artery, where, in the 

 adult, the definite and permanent lymphatic trunks closely 

 resemble in their arrangement the peri-omphalo-mesenteric 

 annular veins of certain reptilian embryos. 



