Johnston, Forebrain Vesicle in Vertebrates. 495 



caudally by the tuberculum posterius. Later, when the hypophysis 

 Has reached its definitive position a saccus outgrowth from the 

 inferior lobes appears and the region at which it is connected with 

 the brain may be called the infundibulum. There are therefore in 

 the floor of the forebrain vesicle four recesses formed as in selachians : 

 terminal pit or preoptic recess, primitive optic groove or postoptie 

 recess, infundibulum and mammillary recess. 



The fonnation of the velum transversum, epiphysis and para- 

 physis need not be described as they are already known (Minot and 

 others). These structures are represented in Figs. 30, 31, 32, 33. 



Fig. 33. Amblystoma p., all the chief features of the forebraiu developed. 

 Median sagital section, x 40. 



5. Beptiles and Birds. 

 I have to say here only that the study of whole mounts of chick 

 embryos between 20 and 40 hours of incubation shows that the same 

 relations exist in the region of the optic chiasma as in Squalus and 

 Amblystoma. In early embryos the optic vesicles are connected 

 by a primitive optic groove behind the terminal ridge. Later, the 

 optic ridge is formed, the terminal pit becomes connected with the 

 cavities of the optic stalks, and the optic chiasma occupies the 

 terminal ridge. 



6. Mammals. 

 The chief peculiarity of the mammalian brain is the great size of 

 the cerebral hemispheres. In the adult, as is well known, there is 



