BASIS OF THE CELL THEORY. 283 



It was not till 1854 that I obtained proof in the frog by 

 the use of hardening agents^ that the latter is the mode ; that 

 is to say, that in the process of segmeutationj the firm cell- 

 membrane becomes constricted, and sends equally solid dis- 

 sepiments inwards into the protoplasm. In this way the 

 division is completed, and consequently the new cells are 

 at once formed, fully furnished with a firm membrane. 



The cell membranes, therefore, even in the cases where they 

 remain inseparably united to the protoplasm, or are, as it is 

 said, chemically identical with it, possess more than a mere 

 historical interest (inasmuch as they first afforded the cell 

 theory a certain and intelligible basis) ; but they would in any 

 case so far have to be reckoned among the essential attributes 

 of the embryonic cells, that they establish the fact that 

 all the surfaces of cells represent continuations of that of the 

 ovum. In other Avords, we must comprehend in the definition 

 of an embryonic cell from which tissues are produced, so far 

 as our present knowledge extends, its origination, in ac- 

 cordance with the laws above stated, from a process of division, 

 and not regard any accidental mass of protoplasm furnished 

 with a nucleus as a cell. 



The next question is, whether every kind of cellseform 

 structures in the animal body arise in the manner stated. As 

 regards the normal tissues, no certain exception is as yet 

 known. But, under pathological conditions, the organization 

 of cells within cells in an endogenous way, has been placed 

 beyond doubt by the observations of Hiss, Buhl, Weber, 

 and myself. 



Now, since this endogenous cell formation in which the 

 nucleus of the parent cell has no part, does take place, it 

 is quite conceivable also that in the course of fibrous tissues, 

 which are known to be the production and prolongations 

 of cells, cellseform structures may also occasionally arise. 

 It would be even possible to ascertain — and this remains to be 

 done — whether the same thing may not also take place in many 

 normal tissues. I would remark that in my papers on ^'cell for- 

 mation " (in Mliller^s Archiv, 1852,) I have pointed out, as 

 striking instances, the secondary prolongations of the vessels in 

 the cutis in the frog, as structures in which I had been unsuc- 

 cessful in referring the nuclei which made their appearance 

 to a division of the embryonal nuclei, any more than ithas 

 hitherto been possible to trace any connection between the 

 stellate cells (Virchow^s " connective tissue-corpuscles ") in 

 fully formed connective tissues and the embryonal cells. 



It would be one of the most interesting discoveries, if we 

 could show that not only in pathological conditions which 



