THE FACTORS OF ORGANIC EVOLUTION. 199 



verted layer, or hypoblast, are developed the alimentary canal along 

 with those parts of its appended organs, liver, pancreas, &c, which 

 are concerned in delivering their secretions into the alimentary canal, 

 as well as the linings of those ramifying tubes in the lungs which con- 

 vey air to the places where gaseous exchange is effected. And from 

 the mesoblast originate the bones, the muscles, the heart and blood- 

 vessels, and the lymphatics, together with such parts of various internal 

 organs as are most remotely concerned with the outer world. Minor 

 qualifications being admitted, there remain the broad general facts, 

 that out of that part of the external layer which remains permanently 

 external, are developed all the structures which carry on intercourse 

 with the medium and its contents, active and passive ; out of the in- 

 troverted part of this external layer, are developed the structures which 

 carry on intercourse with the quasi-external substances that are taken 

 into the interior solid food, Water, and air ; while out of the meso- 

 blast are developed structures which have never had, from first to last, 

 any intercourse with the environment. Let us contemplate these gen- 

 eral facts. 



Who would have imagined that the nervous svstem is a modified 

 portion of the primitive epidermis? In the absence of proofs fur- 

 nished by the concurrent testimony of embryologists during the last 

 thirty or forty years, who would have believed that the brain arises 

 from an infolded tract of the outer skin, which, sinking down beneath 

 the surface, becomes imbedded in other tissues and eventually sur- 

 rounded by a bony case ? Yet the human nervous system in common 

 with the nervous systems of lower animals is thus originated. In the 

 words of Mr. Balfour, early embryological changes imply that 



" the functions of the central nervous system, which were originally taken 

 by the whole skin, became gradually concentrated in a special part of the skin 

 which was step by step removed from the surface, and has finally become in the 

 higher types a well-defined organ imbedded in the subdermal tissues. . . . The 

 embryological evidence shows that the ganglion-cells of the central part of the 

 nervous system are originally derived from the simple undifferentiated epithelial 

 cells of the surface of the body." * 



Less startling perhaps, though still startling enough, is the fact that 

 the eye is evolved out of a portion of the skin ; and that while the 

 crystalline lens and its surroundings thus originate, the "percipient 

 portions of the organs of special sense, especially of optic organs, are 

 often formed from the same part of the primitive epidermis" which 

 forms the central nervous system, f Similarly is it with the organs 

 for smelling and hearing. These, too, begin as sacs formed by infold- 

 ings of the epidermis ; and while their parts are developing they are 

 joined from within by nervous structures which were themselves epi- 

 dermic in origin. How are we to interpret these strange transforma- 

 tions ? Observing, as we pass, how absurd from the point of view of 



* Balfour, I.e. Vol. ii, p. 400-1. f Ibid., p. 401. 



