THE EVOLUTION OF THE BRAIN lOQ 



This truly remarkable growth of the central nervous 

 system of man and other vertebrates from the ectoderm of 

 the embryo is of great significance when the stages in the 

 evolution of the nervous system in the lower animals are 

 recalled. It will be remembered that in the simplest animals 

 in which a nervous system occurs, the sea anemones, the 

 coral animals, and the hke, this system is a part of the 

 outer skin. This condition, it will be recalled, is also reahzed 

 in certain worms, but in others the central nervous organs 

 have broken away from the outer skin and have migrated 

 into a deeper situation where they regularly occur in crabs, 

 insects, snails and other higher animals. That is to say the 

 brain and spinal cord in the developing vertebrate repeat a 

 series of changes that is seen in the successive evolutionary 

 steps in the lower animals. They illustrate an important 

 principle in embryology, namely, the so-called law of 

 recapitulation which may be stated briefly as follows: 

 in the development of any of the higher animals the creature 

 passes temporarily through stages that are permanent con- 

 ditions in the lower forms. Thus in the early stages of 

 embryonic growth in vertebrates the nervous system is 

 temporarily a part of the outer skin, a condition that is per- 

 manent for this system in sea anemones, coral animals, 

 and others of the same general type. 



The evolution of the vertebrate brain takes place on lines 

 quite different from those followed by the cord. This is well 

 seen in the sensory relations of the two structures. So far 

 as our conscious life is concerned the spinal cord has to do 

 chiefly with the sensory impulses from the skin. These 

 impulses enter the cord, excite reflexes or other types of 

 movement, and make their way to the brain to call forth 

 appropriate sensation such as touch, pain, cold, hot, and the 

 like. Although these sensations are in reality resident in 

 the brain itself we refer them to the stimulated spots in the 

 skin. If we prick the end of a finger with a needle, we have a 

 sensation of pain and we think of the pain as resident in the 

 tip of the finger though as a matter of fact it is in the brain. 

 This is the common rule for most cord sensations. They 

 are referred by us to the spot on the surface of the body 

 where the stimulation occurred. This reference is not always 



