TISSUES. 



365 



Zoophytes a middle layer of connective tissue develops 

 between the two priiiiaiy gevm-layers, it is only in the 

 most highly developed Plant Animals that muscle and nerve 

 tissue is formed. As has akeady been said, the latter first 

 appeared as a common nerve and muscle tissue {ncuvo- 

 ■musculum, Fig. 293; cf p. 358). It was only afterwards 

 that the muscle-tissue (Fig. 295) separated from the nerve- 

 tissue (Fig. 294). The greater part of the nerve-tissue is 

 derived from the skin-sensory layer, the greater part of the 

 muscle-tissue from the skin-fibrous layer. 



Vascular tissue (vK.^aliurii) must be regarded as forming 



Fig. 29G. — Vascular tissue (vasalium). A hair- vessel from the 

 mesentery : a, vascular cells ; 6, the kernels of these (" endothehum "). 



Fic. 297. — Red blood cells (corpuscles) of various Vertebrates (equally 

 magnified); 1, Human; 2, Camel ; 3, Pigeon ; 4, Proteus (p. 129); 5, Water- 

 salamander (Triton) ; 6, Frog ; 7, Fish {Cobitis) ; 8, Lamprey (retromyzon) ; 

 a, surface view ; i, edge view. (After Wagner.) 



