I 6 CELL-FORMATION. BLOOD-GLOBULES, [sect. 9,10. 



the most important thickenings of the animal-cells take place on the ex- 

 ternal side of the proteine membrane, seeing for example, that the original 

 cell-membrane is still to be met with within the ossified cartilage-cells. 



§ 9. Formation of Cells. — With regard to the formation of cells, 

 a distinction has hitherto been made between the free origin of 

 them, and their production through the intervention of other cells. 

 The farther, however, investigation is prosecuted, the occurrence 

 of a free cell-formation becomes more and more doubtful ; and it 

 appears that all animal cells only arise, as in plants, in dependence 

 upon other pre-existing cells. In this process of cell-multiplica- 

 tion, it is pre-existing cells which either produce secondary cells, 

 as they are called, or multiply by division — endogenous cell-forma- 

 tion and cell-formation by division. The cell-nuclei always play a 

 very essential part in the multiplication of cells, and appear as the 

 proper centres of formation for their evolution. 



Whilst Schwann, in animals, in contradistinction to plants, regarded the 

 free cell- formation as being the more frequent — that by the intervention of 

 other cells, more as the exception — observers are now coming more and more 

 to the conclusion, that even in this respect animals and plants agree. As for 

 me, I have already long since shown (Entw. d. Cephal. 1844.; Ann. d.sc. Nat. 

 1 846) that in embryos all the tissues are built up of the descendants of the 

 cells which have arisen after the cleaving of the yelk ; and that even in the 

 adult, in the most widely-distributed tissues consisting of cells, as in the 

 cartilages and in horn, free nuclei nowhere occur. Accordingly, I found 

 myself constrained, in the first edition of my German Handbook of Eisto- 

 logy, to limit the free cell-formation very much. Quite recently, Virehou 

 (in Beit.zur. spec.Path.u. Therapie, 1854, p. 329), has. made known a series of 

 facts, from the department of pathological anatomy, which show that in 

 many places where formerly a free cell-formation was admitted, it does r >t 

 occur. If to these facts are added, the new observations of Virchow, wi oh 

 regard to the development of bones, as well as the recent investigations i .to 

 the formation of the lymph-corpuscles, we may indeed conclude, that a -ee 

 formation of cells does not anywhere exist. 



§ 10. Multiplication of Cells by Division, is much more widely 

 Fig. 2. distributed than has hitherto been sup- 



posed; it being highly probable that 

 i'o ) the entire growth of the embryonal 

 y and fully developed cell-tissues, with 

 the exception of cartilage, is solely 

 effected by division. This process is 

 Blood globules of chick, in the act easily observable in free cells suspended 



of division. — Magnified 350 diameters. n • i • .-, -, i t i 



in tluid, as m the red and colourless 

 blood-cells of mammalia, birds, and amphibia. Here, in elongating 



