706 ADDRESS TO THE DEPARTMENT OF ANATOMY 



the primitive epidermis. One of the most interesting of recent 

 embryological discoveries is the fact that the nervous system 

 is, in all but a very few doubtful cases, derived from the epiblast. 

 This fact was made out for vertebrate animals by the great 

 embryologist Von Baer; and the Russian naturalist Kowalevsky, 

 to whose researches I have already alluded, shewed that this was 

 true for a large number of invertebrate animals. The derivation 

 of the nervous system from the epiblast has since been made 

 out for a sufficient number of forms satisfactorily to establish 

 the generalization that it is all but universally derived from the 

 epiblast. 



In any animal in which there is no distinct nervous system, 

 it is obvious that the general surface of the body must be sensitive 

 to the action of its surroundings, or to what are technically called 

 stimuli. We know experimentally that this is so in the case 

 of the Protozoa, and of some very simple Metazoa, such as the 

 freshwater Polype or Hydra, where there is no distinct nervous 

 system. The skin or epidermis of the ancestor of the Metazoa 

 was no doubt similarly sensitive ; and the fact of the nervous 

 system being derived from the epiblast implies that the functions 

 of the central nervous system, whiah 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 

 finally became a well-defined organ in the interior of the body. 



What were the steps by which this remarkable process took 

 place ? How has it come about that there are nerves passing 

 from the central nervous system to all parts of the skin, and 

 also to the muscles ? How have the arrangements for reflex 

 actions arisen by which stimuli received on the surface of the 

 body are carried to the central part of the nervous system, 

 and are thence transmitted to the appropriate muscles, and cause 

 them to contract ? All these questions require to be answered 

 before we can be said to possess a satisfactory knowledge of 

 the origin of the nervous system. As yet, however, the know- 

 ledge of these points derived from embryology is imperfect, 

 although there is every hope that further investigation will render 

 it less so? Fortunately, however, a study of comparative anatomy, 

 especially that of the Coelenterata, fills up some of the gaps left 

 from our study of embryology. 



