272 



G. R. DE BEER 



is plain. Text-figs. 11 to 18 are of Petromyzon fliivia- 

 t i 1 i s , the conditions in p 1 a n e r i are similar. 



At a later stage the hypophysial cavity acquires more definite 

 walls, and dorsally the wall separates itself from the glands 

 (Text-fig. 18). The pars intermedia is the first to lose connexion, 

 a band of connective tissue passing between it and the cavity 



Text-fig. 17. 



Och 



Petromyzon fluviatilis. Ammocoete larva more highly 

 magnified, showing the relations of the hypophysial cavity, the 

 pars anterior, pars intermedia, pars nervosa, and' Uebergangsteil'. 



and also between it and the pars anterior separating it from 

 that also. The pars intermedia, practically devoid of blood- 

 vessels, is now in intimate connexion with the pars nervosa of 

 the infundibulum, and its condition is now similar to that 

 which it presents in the adult. The pars anterior is highly 

 vascular. The cavity becomes more spacious and extends 

 ventrally and posteriorly. It acquires a connexion with the 

 exterior through the hypophysis depression by a split which 

 occurs along the hypophysis strand. In the adult (Text-figs. 19, 

 20, and 21) the separation of the glands from the hypophysial 

 cavity has gone further and they are now firmly encapsuled 



