ON THE CELL THEOET. 93 



On the Inadequacy of the Cell Theory, and on 

 the Early Development of Nerves, parti- 

 cularly of the Third Nerve and of the Sympa- 

 thetic in Elasmobranchii. 



By 



Adam Sedgrwick, F.R.S. 



It is now more than ten years ago since I first pointed out 

 the inadequacy of the cell theory. That I did so in a very 

 guarded manner need hardly be said; but now, after ten 

 years of mature work, I feel justified in giving a stronger 

 expression to the views which I then formed, and which all 

 my subsequent work has amply confirmed. My words then 

 (in 1883) were as follows : — " In short, if these facts are 

 generally applicable, embryonic development can no longer 

 be looked upon as being essentially the formation by fission 

 of a number of units from a single primitive unit, and 

 the co-ordination and modification of these units into a 

 harmonious whole. But it must rather be regarded as a 

 multiplication of nuclei and a specialisation of tracts and 

 vacuoles in a continuous mass of vacuolated protoplasm." 

 Again, in 1888, in the preface to my "Monograph on the 

 Development of the Cape Species of Peripatus,"^ I wrote: 

 " It would appear, indeed, that in Peripatus the cells of the 

 adult, in so far as they are distinct and sharply marked off 

 structures, are not, as appears to be generally the case, present 

 in the earliest embryonic stages, but are gradually evolved as 

 development proceeds. In other words, the cell-theory, if it 



1 ' Studies from the Morphological Laboratory of the University of 

 Cambridge,' vol. iv, part 1. 



VOL. VI. 11 



