DEVELOPMENT OF THE NERVOUS SYSTEM. 



49 



respectively. They remain relatively thin throughout life. As 

 the spinal cord grows it expands laterally and ventrally at the sides 

 of the floor-plate, so that the floor-plate comes to form the bottom 

 of a deep groove which was mentioned in the last chapter as the 

 ventral fissure. At the mid-dorsal line such an open fissure does 

 not appear but it is represented by a septum of connective tissue. 

 Two other zones which probably have much greater significance 

 are marked by a groove which appears on the inner face of each 

 lateral half of the cord. This pair of grooves causes a widening 

 of the central canal and a thinning of the wall of the tube, so that 

 each half of the cord is divided into a dorso-lateral and a ventro- 

 lateral zone or column. These zones were first brought to notice 

 by His, who described them in the human embryo. There is 



Tr. opticus 



Fig. 25. — A part of a transverse section of the optic lobe of the sturgeon. The 

 cells farther removed from the ventricle are more highly developed. 



reason to suppose that the dorso-lateral zone includes the two 

 sensory columns to be described in later chapters and that the 

 ventro-lateral zone includes the two motor columns. 



Observations on these zones in embryos of other vertebrates 

 are much to be desired. 



The DEVELOPMENT OF THE PERIPHERAL NERVES. — The develop- 

 ment of the sensory nerves is a very complex matter, especially 

 in the case of the cranial nerv^es, and an account of its main out- 

 lines only can be given. It was said above that the neural crest 

 4 



