io8 



HUMAN BIOLOGY 



The cavity of the anterior part of the medullary tube, 

 as might be expected, changes eventually into the series 

 of brain ventricles. In this way the spinal cord and brain 



B 



Fig. 4. Three stages in development of central nervous organs as seen in 

 sections across axis of embryo frogs. 



A. Early stage showing medullary plate, p, as thickening in ectoderm, e. 



B. Later stage showing depression of thickened medullary plate to form 

 medullary groove, G. 



C. Final stage showing completed medullary tube, t, separated from outer 

 ectoderm or embryonic skin, e. 



of the adult frog develop from the outer skin or ectoderm of 

 the embryo and migrate into their fmal position in the 

 deeper part of the body. What has been said of the develop- 

 ment of the central nervous organs in the frog holds true 

 for all other vertebrates, man included, for even in us these 

 organs have an external origin. 



