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Anatomy and Development of Veins of Chelonia 



PHALOME£ENTEHiC 



and subcardinal veins also become more important. The subcardinal 

 veins unite with the eandal vein as soon as it is formed and they become 

 the advehent veins of the primitive kidneys. Fig. 11 is a reconstruction 

 of the postcard inal and subcardinal veins of a 9 mm. turtle embryo. 

 Onlv the direct connections between these two sets of veins are shown. 

 The sinusoidal network of the mesonephroi is very complex at this stage. 

 The postcardinals have now reached their maximum of development in 

 their cranial portions, just as have the subcardinals in the caudal re- 

 gion. In other words the primitive portal system of the mesonephroi 

 is now at its height of development. 



r^ppj g-» At about this time a series of changes takes 



fe 'j— ~|—AORT. place which ultimately results in the complete 



M ^ I reversal of the primitive portal system of the 



K« ■z?^ j-t-i.3CL.v.:. mesonephroi. 



These changes are made possible by two im- 

 portant formations: (a) the right hepatic vein, 

 and (b) the caval mesentery. The formation 

 of the right hepatic vein has already been dis- 

 cussed in an earlier part of this paper. 



The development of the caval mesentery has 

 been frequently described by a number of in- 

 vestigators, Hoifmann, Hochstetter, and others, 

 so that it is not necessary to take it up at this 

 time. It arises in the cranial region of the body 

 as a bud or process from the median mesentery 

 and grows both ventrally and caudally so that 

 it becomes attached to the dorsal surface of the 

 liver further and further caudad. When it 

 reaches a certain point slightly crania d of the 

 origin of the omphalo-mesenteric artery, a short 

 vein is developed along its caudal border, which 

 unites the right subcardinal with the right 

 hepatic vein and thus completes the formation 

 of the anlage of the postcava. The short vein formed in the caval 

 mesentery seems to grow from the right subcardinal toward the liver, 

 since in the 10 mm. stage, as noted in an earlier part of the paper, it may 

 be traced up to the border of the liver where it apparently breaks up into 

 a number of smaller vessels. It appears to later form a path for itself 

 through the sinusoids of the liver and thus join the right hepatic vein. 



In the meantime the two subcardinal veins have become connected by 

 several anastomosing branches which at first are small and of about equal 



Fig. 11. Frontal lecon- 

 structioti of the poistcardinal 

 and subcardinal veins of a 9 

 mm. embrj'o of Kini»<te)noit 

 pennsylvanicum. Ventral 

 view. 



