VIII.] 



THE COMMON FROG. 



119 



er. 



Fig. 73.— Diagram illustrating the progressive Changes that take place during suc- 

 cessive stages of the Development of the Brain of Man. i. The brain in its very 

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

 tinuous with the wide cavity {d) of the primitive spinal marrow (;«). The 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, (^pt) below. The wall at the anterior end of the first vesicle (or 

 fore-brain) is the lamina terminalis (t). 3. This figure shows the cerebrum {cr) 

 budding from the first vesicle, its anterior part {o) being prolonged as the olfactory 

 lobe (the so-called olfactory nerve), the cavity of the cerebrum (or incipient lateral 

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

 of the first cerebral vesicle (third ventricle) behind. The latter communication 

 takes place through the foramen of Monro. The walls of the three primitive 

 vesicles are becoming of unequal thickness, and the cavity (1^) of the middle 



