THE EMBRYOLOGY OF THE OPOSSUM 



61 



medullary plate and the neural crest. Miscellaneous details. 

 Comparative notes. 



The heart tubes and the first blood vessels. In two embryos 

 which by somite number and general appearance should be- 

 long to my stage 24 Selenka was unable to find any blood 

 vessels, though he described the hearts with their endothelial 

 tubes. As he was also unable to recognize sensory anlagen in 

 this material, it must be assumed that fixation was bad, or that 

 many of the sections were lost, or that some other such acci- 

 dent made the description of the internal anatomy of these 

 specimens incomplete. 



Fig. 19 Photographs of external views of stage 24. A, dorsal view of embryo 

 and area vasculosa. B, dorsal view of whole vesicle. C, ventral view of embryo 

 and area vasculosa. 



In the six sectioned specimens in The Wistar Institute col- 

 lection which belong to this stage there is a considerable 

 vascular system. The hearts contain endothelial tubes which 

 communicate anteriorly with a capillary plexus running 

 through the mandibular mesenchyme, and posteriorly with 

 a vitelline plexus from the yolk sac (fig. 20, totomount and 

 section L). The mandibular plexus on each side connects 

 medially with a dorsal aorta, also somewhat plexiform in 

 early specimens, which runs caudally beneath the medullary 

 plate. At the level of the first few somites the two aortae are 



