THE EMBRYOLOGY OF THE OPOSSUM 101 



the number of channels, and by stage 27 there are only the two 

 well-organized second aortic arches at this level. The vitel- 

 line veins, the primary head vein, and the umbilical veins as far 

 caudad as the anterior limb ridge, have also passed through 

 similar capillary stages and by stage 27 have likewise be- 

 come organized into single large channels. The point to be 

 emphasized here is that all this has occurred before there 

 has been any circulation of the blood. 



The theory that all blood vessels are differentiated from 

 primitive capillary plexuses seems first to have been pro- 

 posed by Aeby in his *Der Bau des menschlichen Korpers' 

 (1868). In 1893 Thoma (Untersuchungen iiber die Histo- 

 genes and Histomechanik des Gefasssystems) suggested that 

 the enlargement of certain channels and the neglect and 

 gradual elimination of others were due to the circulation of 

 the blood the hydrodynamic theory. When Evans ( '09 b, 

 from which the above two references are cited) established 

 the soundness of the morphological facts, he did not concern 

 himself with the question of causation. Thoma 's theory of 

 the mechanism is still the only one which has been proposed, 

 and is generally accepted today. Arey, for instance ('30), 

 says, ' ' The selection of appropriate channels from the diffuse 

 capillary bed results both from the action of heritable patterns 

 and the hydrodynamic factors incident to the blood flow" 

 and " .... those capillaries from which the blood flow has 

 been diverted, atrophy." I quote this passage because it 

 should be emphasized that the mention of heritable patterns 

 in no way extends or supplements Thoma 's theory. Heredity, 

 which determines all possible responses of the cells con- 

 cerned, forms the basis of the process no matter what 

 mechanical factors play a part in eliciting or provoking the 

 special response which actually does occur. As heredity, then, 

 in a general sense, is the basis of any and all possible ex- 

 planations of the process, the only specific factor which has 

 been suggested is the blood flow. 



The experimental work of Oppel and Roux ( '10) seems to 

 have shown quite definitely that after the blood begins to 



