DEVELOPMENT OF ELASMOBRANCH FISHES. 163 



muscle-cell, would remain, and when the rapid developmental 

 growth had ceased, the two would become united again by the 

 growth of the process which had previously been ruptured. It 

 will be seen that this hypothesis, which I have considered only 

 with reference ta a single nerve and muscle-cell, might be 

 extended so as to apply to a complicated central nervous system 

 and peripheral nerves and muscles, and also could apply equally 

 as well to the sensary as to the nK>tor terminations of a nerve. 

 In the case of the sensory termination, we should only have to 

 suppose that the centre nervous cell became more and more 

 separated by the general growth from the recipient terminal 

 sensory cell, and that during the general growth the connection 

 between the twa was mechanically ruptured but restored again 

 on the termination of the more rapid gTOwth. 



As the descendants of the animal in which the rupture 

 occurred became progressively more complicated, the two ter- 

 minal cells must have become widely separated at a continu- 

 ally earlier period, till finally they may have been separated 

 at a period of development when they were indistinguishable 

 from the surrounding embryonic cells ; and since the rupture 

 would also occur at this period, the primitive junction between 

 the nerve-centre and termination would escape detection. The 

 object of this hypothesis is to explain the facts, so far as they 

 are known, of the development of the nervous system in Verte- 

 brates. 



In Vertebrates we certainly appear to have an outgrowth 

 from the nervous system, which eventually becomes united 

 with the muscle or sensory terminal organs. The ingenious 

 hypothetical scheme of development of the nerves given by 

 Hensen^ would be far preferable to the one suggested if it 

 could be brought into conformity with the facts. There 

 is, however, at present no evidence for Hensen's view, as he 

 himself admits, but considering how little we know of the 

 finer details of the development of nerves, it seems not im- 

 possible that such evidence may be eventually forthcoming. 

 The evidence from my own observation is, so far as it goes, 

 against it. At a time anterior to the outgrowth of the spinal 



1 Virchow's Archiv, Vol. xxxi. 1861. 



