TISSUES. 



365 



Zoophytes a middle layer of connective tissue develops 

 between the two primary germ-layers, it is only in the 

 most highly developed Plant Animals that muscle and nerve 

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

 appeared as a common nerve and muscle tissue (neuro- 

 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 (vasalium) must be regarded as forming 



Fig. 296. — Vascular tissue (vasalium). A hair- vessel from the 

 mesentery : a, vascular cells ; b, the kernels of these (" endothelium "). 



Fig. 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 (Petromyzon) ; 

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



