DEVELOPMENT OF THE LITER. 



777 



the mouth. As the development of the gland advances, 

 the canal becomes more and more ramified, increasing at 



Fig. 234.* Fig. 235^ 



the expense of the blastema in which it is still enclosed. 

 The branches or salivary ducts constitute an independent 

 system of closed tubes (fig. 235). The pancreas is developed 

 exactly as the salivary glands. 



The liver in the embryo of the bird is developed by the 

 protrusion, as it were, of a part of the walls of the in- 

 testinal canal, in the form of two conical hollow branches 

 which embrace the common venous stem (fig. 236). The 

 outer part of these cones involves the omphalo-mesenteric 

 vein, which breaks up in its interior into a plexus of 

 capillaries, ending in venous trunks for the conveyance of 

 the blood to the heart. The inner portion of the cones 

 forms the cellular structure of the organ into which the 



* Figi 234. First appearance of the parotid gland in the embryo of 

 a sheep. 



f Fig. 235. Lobules of the parotid, with the salivary ducts, in the 

 embryo of the sheep, at a more advanced stage. 



