I06 HUMAN BIOLOGY 



nerve strands have separated from the skin and have isunk 

 into deeper situations within the body. In higher forms such 

 as the crabs, insects, snails, and back-boned animals the 

 nervous organs with their growth in size have migrated 

 well away from the skin and occupy positions relatively 

 deep in the body. In other words, the nervous system, at 

 first simple in cellular composition and later complex, 

 migrates from its place of origin, the outer skin, to a deep 

 situation in the animal where it is at once in closer average 

 proximity to the various parts it has to serve and where 

 also it gains protection from external injury. 



In the embryonic growth of the human being few changes 

 are more interesting and significant than those shown by the 

 spinal cord and brain. These organs in the adult are deeply 

 imbedded in the interior of the body and yet, when their 

 development is followed, they are found to arise in a very 

 different region. The general changes seen during the origin 

 and growth of these parts in man are common to all verte- 

 brates and in fact are seen more clearly and easily in many 

 of the forms lower than man. 



If the developing egg of the common frog is watched from 

 hour to hour the beginnings of the spinal cord and brain 

 and their gradual growth and migration can be followed with 

 great certainty. This is more easily accompHshed in an 

 animal whose egg develops freely outside the body as the 

 frog's egg does, than in one whose embryonic growth takes' 

 place within, as in man. The rate of development in the 

 frog is largely dependent upon the temperature of the water 

 in which the eggs are immersed, but in ordinary spring 

 weather the first traces of the brain and spinal cord in the 

 frog's eggs begin to appear about two days after the eggs 

 have been laid and fertihzed. Once the nervous system has 

 started to form its growth is relatively rapid. Within a 

 day or so after its first appearance it is well advanced in its 

 separation from the outer skin and on its path of inward 

 migration. 



It is important for our present purpose to follow briefly a 

 few of the details of this developmental process. The egg of 

 the frog, before any trace of central nervous organs can be 

 seen in it, is a small sphere composed of many cells which 



