CHAPTER III 



THE GERAI-CELLS 



" Not all the progeny of the primary impregnated germ-cells are required for the forma- 

 tion of the body in all animals; certain of the derivative germ-cells may remain unchanged 

 and become included in that body which has been composed of their metamorphosed and 

 diversely combined or confluent brethren; so included, any derivative germ-cell may com- 

 mence and repeat the same processes of growth by inhibition and of propagation by spon- 

 taneous hssion as those to which itself owed its origin; followed by metamorphoses and 

 combinations of the germ-masses so produced, which concur to the development of another 

 individual." Richard Owen.i 



" Es theilt sich demgemass das befruchtete Ei in das Zellenmaterial des Individuums und 

 in die Zellen fiir die Erhaltung der Art." M. NussBAUM.^ 



The germ from which every living form arises is a single cell, de- 

 rived by the division of a parent-cell of the precedmg generation. 

 In the unicellular plants and animals this fact appears in its simplest 

 form as the fission of the entire parent-body to form two new and 

 separate individuals like itself. In all the multicellular types the 

 cells of the body sooner or later become differentiated into two groups 

 which as a matter of practical convenience may be sharply distin- 

 guished from one another. These are, to use Weismann's terms : ( i ) 

 the somatic cells, which are differentiated into various tissues by which 

 the functions of individual life are performed and which collectively 

 form the "body," and (2) \X-\<i gcnn-cclls, which are of minor signifi- 

 cance for the individual life and are destined to give rise to new 

 individuals by detachment from the body. It must, however, be borne 

 in mind that the distinction between germ-cells and somatic cells is 

 not absolute, as some naturalists have maintained, but only relative. 

 The cells of both groups have a common origin in the parent germ- 

 cell ; both arise through mitotic cell-division during the cleavage of 

 the ovum or in the later stages of development ; both have essentially 

 the same structure and both may have the same power of develop- 

 ment, for there are many cases in which a small fragment of the body 

 consisting of only a few somatic cells, perhaps only of one, may give 



1 Parthenogenesis, p. 3, 1849. 

 '^ Arch. Mil;. Anat. XVIII. p. 112, iSSo. 

 SS 



