108 



THE EARLY DEVELOPMENT OF MAMMALS. 



found in human adults, and in such cases the dorsal duct has been commonly 

 termed "accessory." 



The cords of pancreatic cells are at first solid, but they for the most part 

 acquire lumina, thus becoming epithelial gland-tubes. The areas of Langerhans 

 are small patches of pancreatic cell cords found in the adult without any lumina. 



FIG. 62. PIG EMBRYO OF 20 MM. SERIES 60. WAX RECONSTRUCTION or THE DUODENUM AND PANCREATIC 



ANLAGES BY F. W. THYNG. 



Div, Duodenal diverticulum. D.chol, Ductus choledochus. D.panc.d, Ductus pancreatis dorsalis. D.pqnc.v, 

 Ductus pancreatis ventralis. Pane. ace, Pancreas accessor! um (anomaly). Panc.d, Pancreas dorsale. Panc.v, 

 Pancreas ventrale. St, Stomach, x, Ventral process of the dorsal pancreas, on the right of the portal vein. 

 X 55 diams. 



The Excretory Organs. 



No less than three distinct excretory organs are known to occur in vertebrates. 



Of these, the first is termed the pronephros, or head kidney, on account of its 

 position toward the head and in the neighborhood of the heart. It is well developed 

 and the only excretory organ in certain fishes and in the early larval stages of 

 amphibia. In elasmobranchs, which occupy in this respect an exceptional position, 

 and in amniota it exists in a rudimentary form only, except as to its duct, which 

 plays an important role in the further development. The pronephros consists of 



