Morphology of Organs. 147 



interrupted by fibrous partitions. In the higher vertebrates the 

 muscular layers are more thoroughly split up into separate 

 muscles. In vertebrates, finally, we get formed out of the- 

 mesoblast of the body-wall, as well as out of the splanchno- 

 pleuric mesoblast in the head region, the skeleton which has 

 been already described in three vertebrate types, and need not 

 be again described here.^ 



Nervous System. 



The nervous system is entirely a product of the outer layer 

 of the embryo, the epiblast. As a rule, however, much of it 

 becomes removed deeper within the body, leaving only its ter- 

 minal sensory endings in the outer layer of the body ; and even 

 these — as in the case of the retina of the eye, for instance — may 

 also be removed from the surface. The nervous system of the 

 higher animals consists of the central nervous system, which is a 

 chain of ganglia or a cord containing numerous ganglion cells, 

 and of a peripheral nervous system consisting of the nerves which 

 arise from the central nervous system. The latter invariably 

 end in sensory cells situated in the skin or elsewhere, in 

 delicate sensory plexuses, or in muscle fibres. Hence the 

 nerves of the peripheral nervous system may be distinguished 

 into sensory and motor. In the simpler animals there is 

 the rigid distinction referred to between the central and the 

 peripheral nervous system. It is only the central nervous 

 system where ganglionic cells occur. But in higher animals 

 peripherally situated ganglia are to be found, such as the 

 plexuses of cells and fibres connected with the alimentary 

 system of vertebrates. 



As would be imagined on a priori grounds, a general 



' It must be remembered that the tissues formed between the epiblast 

 and the hypoblast arise from at least two distinct sources, in the opinion 

 of some embryologists ; there are the mesoblastic somites (p-olovertebm), 

 (p. 132), and the mesenchyme, budded off as wandering cells from both 

 hypoblast and epiblast. From the latter is formed inter alia a considerable 

 portion of the skeletal elements. If this distinction can really be drawn, 

 some of the above comparisons will fall to the ground. 



