ELEMENTS OF STRUCTURE. 93 



existed were as to the prevalence of one or other of them. The occurrence 

 of free nuclei was taken as conclusive evidence of the pre-existence of 

 these structures, although all were obliged to admit that they might have 

 been liberated by destruction of the body of the cell. And, indeed, the 

 occurrence of cells in such fluids as lymph, mucus, and pus, appeared to 

 be capable of most plausible explanation by means of this theory of free 

 origin; and proliferation of already existing cells, which could not rightly 

 be denied, was yet regarded as exceptional. Tis true that this spontaneous 

 generation (Urzeugung) of animal as compared with vegetable cells, which 

 spring from others already in existence, presents a strange contrast be- 

 tween the construction of the animal and vegetable systems. But, on the 

 other hand, the rapid development of pathological histology based on 

 Schicann's work, seemed to give weight to the theoretical views of this 

 talented man in this department also. The organisation of exudations, 

 the formation of tumours, &c., were interpreted in this manner, to the 

 support of the theory of spontaneous origin. 



On Remak'sr demonstrating in a most elaborate manner that no sponta- 

 neous formation of cells takes place in the embryos of mammalia, but that 

 all new elements have their origin solely from division of previously exist- 

 ing ones, this theory of the generatio cequivoca of animal cells became 

 untenable, as regards the construction of the embryonic body at least. 

 Great exertions were also made by Virchow to prove that for the growth 

 of pathological tissues also (a far more difficult and obscure subject), 

 spontaneous generation of the cell need not exist; and giving up all earlier 

 theoretical conclusions, he conducted his case with the happiest results. 

 A review, likewise, of the earlier investigations touching the cellular 

 tissues of the healthy mature body, showed likewise a scarcity of free 

 nuclei at those points where cells were being formed anew, and led to 

 another easy interpretation, also, of the existence of cells without mem- 

 branes. And now that earnest search began to be made for those already 

 mentioned examples of cell multiplication which had been up till then so 

 rarely met with, they were found to exist in far greater number than was 

 at first supposed. 



This, then, may be regarded a turning-point in histological science. 

 Histologists, as a rule, have now abandoned the theory of the origin of 

 cells without parents, accepting in its stead that of formation alone from 

 others previously existing, though, indeed, still to a certain extent in the 

 light of an article of scientific faith, it must be confessed. The proof, 

 namely, that spontaneous generation of cells in the system does not 

 exist, is not even in the present day capable of being established by 

 facts. And, indeed, it is probable that we never shall be able to adduce 

 proof that such spontaneous generation does not take place in the midst 

 of some of the more inaccessible tissues of the living body. 



And now at the present moment, remembering the former state of 

 science, and how for decades these theories of Schwann were clung to 

 generally, and with an amount even of wantonness, we would inculcate 

 caution. And though everything now seems to point to the conclusion 

 that spontaneous generation of animal cells does not take place, neverthe- 

 less, it seems desirable, for many reasons, that the old view should still 

 have its defenders as well as the new its opponents. Thus will science 

 be compelled to put forth all her energies for the accumulation of that so 

 indispensable material " facts," in order to set her dogmas upon a firm 

 foundation, and histology can only gain by it. 



