History of a transient nervous apparatus in certain Ichthyopsida, 405 



presence of two distinct nervous systems , of different morphological 

 characters, in the development of one of the lower Vertebrates may 

 be something with all important bearings on our ideas of the nature 

 of animal development. 



It may, perhaps, be in place to consider briefly the particular case 

 dealt with in this memoir. 



In the description of an embryo of 71 mm (page 369) the follow- 

 ing passage was emphasised: — "it is of some significance to note 

 that the embryo is now making for the adult form". This statement 

 certainly calls for amplification and explanation. 



Whilst the transient apparatus of ganglion-cells and nerves is in 

 a state of functional activity, other organs, i. e. those of the embryo, 

 are only in course of development, or, at most, in the early stages of 

 histological differentiation. Obviously there is some mystery to be 

 here explored, a problem to be solved. It is somewhat difficult to 

 explain, without the aid of an extended theoretical discussion, the 

 interpretation, widely divergent from the current one, which the writer 

 puts upon the facts of development — an interpretation withal entirely 

 at variance with either of two subordinate explanations, having as their 

 common starting-point the piscine nature of the developing organism 

 as a whole from the moment that gill-pouches and notochord, to 

 name only these, are formed. 



According to the generally accepted view a skate embryo is a 

 young fish from the moment of formation of its gill-clefts, and this 

 is taken to be true of its whole organisation. From this basis the 

 opinions of embryologists diverge. To some the development is a 

 direct epigenesis in which part is gradually built-up upon part during 

 the ontogeny. With an important reservation this view may be adopted 

 here. To others the embryo is gradually changed from time to time 

 into a form belonging to a higher class; and in this way it becomes 

 transformed without the occurrence of any great break, into the specific 

 form of the parent. In other words it recapitulates the stages of 

 its ancestry in its own development. 



From the standpoint of the writer this can only be admitted to 

 a limited extent concerning the organs, and to the organism as a 

 whole any repetition of the ancestry must be denied. Kegarding the 

 former view, a gradual passage of the embryo to the adult form would, 

 in my opinion, only hold good for the embryo itself apart from its 

 transitory or larval foundation. 



In certain cases, as in the Pilidium development of a Nemertine, 



27* 



