102 EDWARD MCCRADY, JR. 



circulate, it can and does influence the size, course, and muscu- 

 larity of vessels; but all of the development which occurs 

 during the prefunctional period (Huxley's period of 'chemo- 

 differentiation ' ) remains totally unaccounted for. What I 

 call attention to here is that, with one exception, every sort of 

 change which occurs in the circulatory system at all occurs 

 in the prefunctional as well as the functional stage. The one 

 exception is the development of the muscular layer around the 

 vessels. As this occurs only after function begins, there is no 

 histological distinction between arteries and veins in stage 27. 



The reconstruction of stage 27 (fig. 30) shows a number of 

 new details in the vascular system which have not yet been 

 mentioned. The capillary plexus of the third aortic arch is 

 in process of forming. Capillary twigs from the vena capitis 

 medialis can be seen extending laterally around the otocyst. 

 The great venous plexus near the heart represents the proxi- 

 mal ends of the anterior and posterior cardinal veins, the duct 

 of Cuvier, and the paracardinal plexus. The umbilical vein, 

 which enters this plexus from behind and near its ventral 

 end, has been cut off at its mouth to expose the lung bud. 



More or less opposite the mouth of the umbilical a new vein 

 can be seen coming from the ventral part of the head back 

 across the dorsal wall of the pericardium to empty into this 

 plexus from in front. This is the inferior jugular, which is 

 not to be confused with the external jugular or the anterior 

 jugular of mammals. This vein was first described by 

 Grosser ('01) in the embryo of the bat, and later ('07) shown 

 by him to be homologous with the inferior jugular of fishes, 

 amphibians, and reptiles. He also found it in the cat, guinea 

 pig, and human embryos. Lillie ('08) described it in the 

 chick as the external jugular; and Lewis ('09) described it in 

 the human embryo as the linguofacial. It is very well de- 

 veloped in the opossum and remains an independent vessel 

 until stage 32, though meanwhile its mouth migrates up the 

 duct of Cuvier to the internal jugular vein. At this time it 

 also acquires connection with branches from the external 

 jugular, and its flow is soon diverted into the latter channel 



