392 



STRUCTURE OF VERTEBRATES, 



Alimentary System. — Summary. 



Region of the Gut. 



Outgrowths. 



Associated Structures. 



Mouth-cavity, 



or Stomatodeeum, 



or Fore-gut, 

 originating as an epiblastic or 

 ectodermic invagination , or 

 from two gill-clefts. 



Teeth. 



Salivary glands. 



Tongue. 



(Note relation between the 

 mouth and the oral part of 

 the hypophysis.) 



Pharynx, gullet or oaso- 

 phagus, stomach, small in- 

 testine, large intestine, and 

 rectum ;=the mesenteron or 

 mid-gut, originating from the 

 cavity of the gastrula, the 

 archenteron or primitive gut ; 

 lined by endoderm or hypo- 

 blast. 



T^L -J ^ and the gill- 

 Thyroid p^rvisceral- 

 ThymusJ ^j^f^^^ 



Air-bladder ; lungs. 

 Liver. 



Pancreas. 

 Allantois. 



With the several out- 

 growths the surrounding 

 mesoderm or mesoblast be- 

 comes associated often to a 

 great extent. 



(Note also the origin of the 

 notochord as an axial differ- 

 entiation of cells along the 

 mid-dorsal line of the embry- 

 onic gut.) 



Anal Region, 



or Proctodseum, 



or Hind-gut. 



Where the mouth of the 

 gastrula persists, it forms the 

 terminal aperture of the gut, 

 and then there is no ecto- 

 dermic invagination or procto- 

 da:um. 



In some Fishes, all Amphi- 

 bians, all Sauropsida, and the 

 Prototherian Mammals, the 

 terminal part of the gut is a 

 cloaca or common chamber 

 into which the rectum, the 

 urinary and the genital ducts 

 open. 



Body-Cavity. — In Amphioxus the body-cavity or coelome 

 arises as a paired pouch (enterocoele) from the archenteron. 

 It afterwards becomes divided from before backwards into 

 compartments. In other cases, the body-cavity seems to 

 appear as a cleft (schizoccele) in the mesoderm, or it may be 

 formed by some reduced and disguised form of pouching. 



Vascular System. — From Cyclostomata onwards the blood 

 fluid contains red corpuscles, i.e.^ cells coloured with haemo- 

 globin — a pigment which readily forms a loose union 

 with oxygen, and bears this from the exterior (gills or lungs) 

 to the tissues. These pigmented cells are usually oval and 

 nucleated. In all Mammals except Camelidse they are 

 circular. Moreover, the full-grown red corpuscles of Mam- 

 mals have no visible nuclei. The blood fluid also contains 

 nucleated uncoloured amoeboid cells, the white corpuscles 



