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portance of the pronephros the posterior cardiual veins furnished their 

 important blood-supply and that with their atrophy the associated 

 veins, then without their distributional area, diminish greatly in size 

 in the pronephric region. Still these veins form at the same time the 

 efferent vessels of the increasingly important mesonephros, the abund- 

 ant circulation of which must be maintained. At just the same time 

 that the anterior sections of the posterior cardinal veins thus diminish, 

 the vena cava commences to develop and in a comparatively short 

 time becomes a very large vessel extending directly across from the 

 heart to the larger right posterior cardinal vein and receiving its 

 abundant blood-supply. The vena cava, therefore, is to be looked 

 upon as a short cut through which the blood from the mesonephros 

 passes directly to the heart. This explanation of the origin of the 

 vena cava is verified by the fact that it is in those forms which have 

 a highly developed pronephros, namely the Dipnoi and Amphibia, that 

 the cava is first developed, always appearing just as the functional 

 activity of the pronephros begins to decline. In the FJasmobranchs, 

 Ganoids and Teleosts, where the pronephros has only a slight physio- 

 logical value and is of brief existence, the cardinal veins do not break 

 up in it but remain opening freely into the heart, and the vena cava 

 is not developed. 



In order to understand the development of the branchial arteries 

 some attention was given to the development of the gills. The 

 following notes concerning the development of the respiratory system 

 may be added to the foregoing account of the development of the 

 vascular system. 



The gill-pouches are intermediate between those of Pisces and 

 Amphibia; that is, there are six pouches formed in the embryo as in 

 Pisces, but, as in Amphibia, the most anterior of these (hyo-mandi- 

 bular) is never perforated. The gills themselves, however, are typi- 

 cally Amphibian both in character ami arrangement. The gill-filaments 

 are covered with ectoderm. True larval gills are developed similar 

 to those of some Amphibia and not "external" only in the fact that 

 they are never quite long enough to project beyond the large oper- 

 culum. The hyoidean hemibranch of the adult is not indicated in the 

 young fish even nine weeks after hatching: this is similar to the con- 

 dition in the young Lepidosteus. 



The lung develops as a single ventral diverticulum from the wall 

 of the pharynx. It develops slowly and moves over toward the right 

 side of the gut, finally coming to lie partially in the hepatic mes- 



