1895. NOTES AND COMMENTS. 375 



Adam Sedgwick on the Inadequacy of the Cell-Theory. 



So long ago as 1883, when the cell-theory was supreme, 

 Mr. Adam Sedgwick criticised it, and at repeated intervals he has 

 returned to the attack. He took the view that embryonic development 

 was not the formation of cells from a single cell accompanied by co- 

 ordination and modification of these separate units into a harmonious 

 whole. He held that the whole was always co-ordinate and continuous, 

 and that the process of development was "a multiplication of nuclei and 

 a specialisation of tracts and vacuoles in a continuous mass of vacuo- 

 lated protoplasm." In 1888, in his monograph on the development of 

 Peripatus, he insisted that the development did not proceed by the 

 formation of cells. Nuclei multiplied : special tracts were plotted off, 

 and as development proceeded separate cells were marked off. In 

 the Qnavtevly Joiivnal of Microscopical Science (Nov., 1894), ^e draws 

 conclusions of the same nature from his observation of elasmobranch 

 embryos. The so-called mesenchyme tissue in them is always 

 drawn and usually thought of as a system of branched cells lying 

 between the inner and outer layers, and connected with these only at 

 the points where the mesenchyme is supposed to take origin from the 

 outer layer. Mr. Sedgwick assures us that this is not the case : that 

 the whole young elasmobranch is a continuous mass of pale tissue ; 

 on the surfaces the nuclei are arranged in layers, and give an 

 appearance to those on the look-out for epithelia of being distinct 

 epithelia : that the mesenchyme is a net-work of the same funda- 

 mental protoplasm, continuous in itself and with the ectoderm and 

 endoderm, and having nuclei at the nodes. Similarly, he asserts that 

 nerves neither grow out as cell-processes from the neural crest to the 

 periphery nor inwards from the peripher5\ The rival theories are 

 both wrong. It is more correct to say that nerves are simply parts of 

 the continuous ground-substance, which become specialised by the 

 intrusion of nuclei. As it happens, save in the case of the third 

 nerve, the specialisation and entrance of nuclei appear from the 

 centre towards the periphery, from the neural crest outwards. The 

 nerves themselves are developments of the mesoblastic reticulum. 



The Nucleus and " its Sphere of Influence." 



Mr. Sedgwick, no doubt purposely, couches his attack in strong 

 words, but as our arguments in the preceding paragraphs show, we 

 are not certain that he is a solitary prophet speaking to an unheeding 

 generation. We agree with him thoroughly that many mistakes have 

 been made by the persistence of the original idea that cells have 

 walls, and that embryologists frequently represent areas around nuclei 

 by lines, where these are not clearly marked off, and that they speak 

 and think of the proliferation of cells, when they mean only the pro- 

 liferation of nuclei. But while the habit has lingered on in drawing, 

 and in many cases in language, in theory and in real interpretation of 



