Johnston, Forehrain Vesicle in Vertebrates. 



493 



primitive optic groove by the terminal ridge. This is a pit formed 

 in the lower part of the nenropore and is the terminal pit (Fig. 25). 

 From the earliest stages after the formation of the neural plate 

 and folds, the region from which the hypophysis will be formed can 

 be accurately located. In a median sagittal section of any stage 

 up to the time when the hypophysis is invaginated, a slight reen- 

 trant angle is seen between the terminal ridge and the preoral ento- 

 derm. The ectoderm of this angle will form the hypophysis. When 

 the neuropore closes this hypophysial ectodenn is slightly thickened 

 and is continuous with the lower border of the thick plate formed 



Fig. 30. Amblystoma p., stage vvheu the parapbysis is formed. Model of 

 the right half of the head seeu from the medial surface. The model was 

 made from a series of sagittal sections which were oblique to the longitudinal 

 axis, so that the surface of the model lies in the median plane at the fore- 

 brain but passes to the left of the middle at the hindbrain. x 40. 



by the fusion of ectoderm and neural tube in the neuropore. When 

 the hypophysis begins to push in it presses on the anterior surface 

 of the preoral entoderm and as the primitive optic groove becomes 

 depressed the brain wall presses on the preoral entoderm from above. 

 In this way the preoral entoderm is pushed back and the hypophysis 

 insinuates itself between the entoderm and the posterior wall of the 

 primitive optic groove as in Squalus. The preoral entoderm be- 

 comes shorter and blunter, but none of it is cut off as in Squalus. 



