DEVELOPMENT. 



33 



blocked by means of a longitudinal fold and many 

 transverse folds; thus many segments or joints are 

 produced, known as myotomes or mesoblastic somites. 

 From these develop bone, voluntary muscle, and the 

 dermis of the skin. 



While these progressive changes are going on in the 

 mesoderm, a longitudinal dorsal groove develops in 

 the ectoderm. By a median fusion of the margins of 

 the groove, it is transformed into a longitudinal canal, 

 the neural canal, which develops into the brain and 

 spinal cord. 



This brief embryonic growth has been productive 

 in specialization and differentiation of cells. In no 

 case will the cells of one germ layer reproduce, re- 

 place, or function for the cells of any of the other 

 layers. As derivatives of these three germ layers we 

 are able to give the following table (Minot, " Em- 

 bryology, " 1903)- 



