DENTITION 



817 



is used ; when the cusps form a crescent, the tooth is called selenodont 

 when there is a long crown with the neck (the junction region between 

 crown and root) deep in the socket, the tooth is called hvpsodont ; 

 when there is a short crown with the neck at the surface of the gum, the 

 term brachvodont is used. 



pm4 



m.i. 



m.2- 



"J.2^ Tn.3 



n t a 



Fig. 490. — Dentition of a dog. — Fiom a Specimen. 



t.i. First incisor ; i.z, second incisor ; ^.3, third incisor ; c, canine ; 

 pm.i, first premolar ; pm.z, second premolar ; pm.^, third pre- 

 molar ; pm.^, fourth premolar; m.i, first molar; m.2, second 

 molar ; fn.3, third molar. The carnassial or sectorial is the last 

 premolar in the upper jaw, the first molar in the lower jaw. The 

 faint line indicates the boundary between the projecting part of 

 a tooth and the rooted part embedded in the jaw. 



Developmentandplacentation. — The ova of Mammals, 

 except Monotremes, are small ; even those of the Whales 

 are " no larger than fern seed." They are formed from 

 germinal epithelium, the cells of which grow inwards in 

 clustered masses into the connective tissue or stroma of the 

 ovary. In each cluster one cell predominates over its neigh- 

 bours ; it becomes an ovum ; the others invest and nourish 

 it, and are called follicle cells. 



In the middle of each clump or Graafian follicle a cavity 

 is formed containing fluid, and into this cavity the follicle 

 cells immediately surrounding the ovum project, forming 

 what is called the discus proligerus. 



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