186 GENERAL EVOLUTION. 



the repetition of a pre-existent structural element ? This is no 

 doubt the case, for, as will be shown beyond, cartilage, though the 

 least cellular of all the tissues, is formed originally by cell repetition 

 or division. Again, the ultimate lobules of the most complex gland 

 are but repetitions of the diverticula of the simply branched, and 

 each of the latter repetitions of the simple cul-de-sac, which has 

 its origin in a convexity of an originally plane surface. This con- 

 vexity is again the result of repetition of cells or cell-division, 

 whereby their number is increased and the surface rendered 

 convex. 



We are thus, in both the solid segment and hollow sack, brought 

 down to cell-repetition. Thus it is with organs, as with entire 

 animals, in which, following the line of simplification, we reach at 

 last forms composed of cells only {Actlnojjhri/s, e. g.) and then 

 the unicellular {AmoBba). 



If this be the origin of organs, the question whether repetitive 

 growth has constructed tissues remains for consideration. 



In growth, each segment — and this term includes the parts of 

 a complex whole or parts always undivided (as the jaw of a whale 

 or the sac-body of a mollusk) — is constructed, as is well known, by 

 cell-division. In the growing foetus the first cell divides its nucleus 

 and then its whole outline, and this process, repeated millions of 

 times, produces, according to the cell theory, all the tissues of the 

 animal organism or their bases, from first to last. That the ulti- 

 mate or histological elements of all organs are produced originally 

 by repetitive growth of simj)le nucleated cells, with various modifi- 

 cations of exactitude of repetition in the more complex, is taught 

 by the cell theory. The formation of some of the tissues is as fol- 

 lows : 



First Change. — Formation of simple nucleated cells from ho- 

 mogeneous protoplasm or the cytoblastema. 



Second. — Formation of new cells by division of nucleus and body 

 of the old. 



Tliird. — Formation of tissues by multiplication of cells with or 

 without addition of intercellular cytoblastema. 



A. In connective tissue, by slight alteration of cells and addi- 

 tion of cytoblastema. 



B. In blood, by addition of fluid cytoblastema (fibrin) to free 

 cells (lymph-corpuscles), Avhich in higher animals (vertebrates) de- 

 velop into blood-corpuscles by loss of membrane, and by cell-devel- 

 opment of nucleus. 



