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G. R. DE BEER 



horizontal in Petromyzon is rotated through 90° and hes 

 anterior and vertical. 



(ii) The down-growth of the infundibulum in connexion with 

 the specialization of the pars nervosa forces the pars intermedia 

 down also so that it lies posterior to the hypophysial cavity. 

 In this manner the hypophysial cavity comes to lie between 



Text-fig. 22. 



c 



Diagram illustrating the difference in relations between the glands 

 and the hypophysial cavity in (a) Petromyzon and (6) higher 

 vertebrates. Pars anterior dotted, pars intermedia lined, pars 

 nervosa black. 



the pars anterior and the pars intermedia. This is shown 

 diagrammatically in Text-fig. 22 ; and it is on such lines that 

 I would explain the difference between the relations of these 

 organs in Cyclostomes and higher forms. 



Swale Vincent (1922) gives a figure of the pars intermedia 

 on both sides of the cleft, i.e. his pars anterior is separated 

 from the cleft by a strip of tissue labelled pars intermedia. 

 If the cleft should represent the true hypophysial cavity, this 



