118 EMBEYOLOGY OF THE LOWEE VEETEBEATES oh. 



myotome and spinal cord have not yet commenced to move apart,' 

 confirmed later in the case of the motor trunks of Elasmobranchs 

 by Paton, and in the case of the olfactory nerve of various 

 Vertebrates by Elliot Smith and others, seems to rest upon a secure 

 basis of observation. It is difficult, therefore, to avoid the 

 expectation that the progress of future research will show such a 

 primitive protoplasmic bridge between centre and end-organ to be 

 the normal forerunner of nerves in general. 



But, if this be so, we are faced by the question as to the actual 

 mode of origin of such bridges and here we pass into a region 

 where direct observation is either impossible or unreliable. Those 

 who accept Hensen's views in their entirety would look upon 

 them as representing intercellular connexions persisting from the 

 earliest segmentation stages. Eeasons have already been given 

 (p. 37) for disbelieving in the persistence of such bridges between 

 the cells of the segmenting egg. The connexion appears certainly 

 to arise at some later period — but exactly when seems to be a 

 question incapable of answer by direct observation. 



When considering these general problems regarding the nervous 

 system it should be borne in mind that the nervous system has for 

 its main purpose the keeping of the various parts of the body linked 

 together into an organic whole, in spite of their increasing differentia- 

 tion and specialization. It has for its function the providing of 

 -exquisitely specialized pathways by which the living impulses can 

 traverse the whole length of a relatively immense body at least 

 as readily as they originally did the minute blob of ancestral 

 protoplasm. 



Bearing in mind this primary consideration will cause one to 

 reflect that the evidence must be overwhelming before one is 

 justified in believing that this organ system, whose most striking 

 functional feature is continuity, has come in the course of evolution 

 to be characterized by the structural discontinuity involved in the 

 neurone theory of adult structure, or in the outgrowth theory of 

 ontogenetic development. 



Again it is important to bear in mind the high degree 

 of probability attached to the view, originated long ago by 

 0. and E. Hertwig (1878), that the nervous system of the higher 

 metazoa, including Vertebrates, has been evolved out of a sub- 

 epithelial nervous network of the kind still seen in some of the more 

 lowly organized groups such as Coelenterata and Echinodernmta. 

 We may suppose that such a plexus was present in the far back 

 ancestors of Vertebrates over the basal surface of both ectoderm and 

 endoderm cells (as in modern Actinians, Havet, 1901) and that nerve- 

 trunks became evolved as local condensations of such a network, 

 just as we still see in the nerve-strands of a Medusa or a Starfish. 



In this connexion, it is of interest to note that according to the 

 protostoma theory of the fundamental structure of the vertebrate 

 body, which will be found stated later, in Chapter IX., the points 



