ON THE CELLULAR TBEORY OF DEVELOPMENT. 87 



On the Inadequacy of the Cellular Theory of 

 Development, and on the Early Development 

 of Nerves, particularly of the Third Nerve 

 and of the Sympathetic in Elasmobranchii. 



By 

 Adam Scdgrwlck, F.R.IS. 



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

 the inadequacy of the cellular theory of development. 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 j wrote: 

 '' It would appear, indeed, that inPeripatus the cells of the 

 adult, in so far as they are distinct and sharply marked o£F 

 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. 



