VIII.] 



THE COMMON FROG. 



119 



; 73.— Diagram Illustrating the progressive Changes that take place during suc- 

 cessive stacres of the Development of the Brain of Man. i. The bram m its very 

 early condition, when it consists of three hollow vesicles the cavity of which is con- 

 tinuous with the wide cavity (O of the primitive spinal marrow im). ihe brain 

 substance forms an envelope of nearly equal thickness throughout. 2. Here the 

 first vesicle or fore-brain has developed the pineal gland (//) above and the 

 pituitary' body, (/O below. The wall at the anterior end of the first vesicle (or 

 fore-brain) is'the lamina terminalis {(). -. This figure shows the cerebrum (fr) 

 budding from the first ve.sicle, its anterior part {0) being prolonged as the olfactory 

 lobe (the so-called olfactory neri-e), the cavity of the cerebrum or incipient ateral 

 ventricle) communicating with that of the olfactory lobe in front and with that 

 of the first cerebral vesicle (third ventricle) behind. 'I he latter communication 

 takes place through the foramen of Monro. The walls of the three primitn-e 

 vesicles are becoming of unequal thickness, and the cavity (p) of tlie middle 



