48 



ANATOMY OF THE CENTRAL NEETOUS SYSTEM. 



the different brain-structures in the animal series were added to the spinal 

 cord, essentially increasing the capability of the organism. 



The amphioxus practically possesses only the spinal cord, corresponding 

 to the metameres, which receives and gives off nerves. "Whatever brain- 

 structures it may have are so small that the investigations of our best men, 

 covering several decades, have only recently — through KupfEer — -succeeded 

 in finding them. The brain-segments of the nervous system of amphioxus 

 has as yet remained quite refractory to physiological experiments. A de- 

 capitated amphioxus conducts himself just like one which still possesses the 



Fig. 18. — Sagittal section through the brain of a four-months' larval 

 sturgeon. (After von KupfCer.) 



pointed anterior end of the head (Steiner). However, all craniate verte- 

 brates possess, anterior to the spinal cord, at least the brain-structures above 

 mentioned. In the field of morphology one would hardly find anything 

 more interesting or instructive than a glimpse of the development of the 

 brain; of the progressive or retrogressive development of particular parts; 

 of the development of higher structures from parts which in other animals 

 still exist as simple cuticle. Let us now become acquainted with these 

 processes step by step, — and see how the whole was gradually built up, how 

 it came to be and how it is ever changing, — here increasing and there- 

 decreasing. 



