Body ill Vertebrates and Artliropods. 



247 



becomes verticallj prolongeJ in transverse section, and the 

 central canal wiiicli it contains also becomes vertically 

 elongated." Then follows the differentiation (1) of the 

 epithelium of the central canal, (2) of the grey matter of the 

 cord, and (3) of the internal coating of white matter. " The 

 white matter is apparently the result of a differentiation of the 

 outermost parts of the superficial cells of the cord into longi- 

 tudinal nerve-fibres, which remain for a long period without 



a medullary sheath The grey matter and the central 



epithelium are formed by a dift'erentiation of the main mass 

 of the spinal cord." 



There thus appears to be a lack of homology in the histo- 

 logical topography and origin of the nervous system in 

 Chordata as compared with the Annelidan worms and the 

 Arthropods. 



The relation of the nervous system of Arthropods is con- 

 stant ; after the stomodaium has been formed, commissures 

 from the brain pass down and connect the latter with the 

 subcesophageal ganglion, which is ventral. This relation of 

 the postoesophageal nervous system to the ventral side of the 



Fi-. 4. 



Relatious of the nervous system of an embryo Ortliopterous insect to the 

 body-walls : hr, braili ; slxj^ siiboesopliageal ganglion ; mj, nervous 

 cord; st, stomodreum ; ;)>•, proctodseum ; mc, nialpighian tubes; 

 mesen, mid-intestine : ht, heart : md, mandibles ; mx, inx' , 1st and 

 2nd maxillae. From Ayers, with changes. 



body is as constant as the disposition of the ventral surface 

 of the embryo of Insects before the revolution of the embryo, 

 or of the embryos of Annelid worms and Crustacea. The 

 position of the Arthropod embryo is the reverse of that of 

 Vertebrates. The vertebrate disposition of the priiuitive 

 nervous system is also seen in the embryo Tunicate (figs. 1, 2). 

 Mori^hology. — The brain of the Arthropoda is contained in 

 a structure' which throughout is lacking in homology with 



