lOO THE GERM-PLASIM 



structure which has to be considered, but the fonnation of 

 a definite number of bones of a definite shape and size, arranged 

 in a definite series, must also be taken into account. What 

 assumptions .must we make in order to explain such an ac- 

 curately prescribed and complex mode of construction ot 

 these parts? If the fore-limb of a newt {Triton) is cut off 

 between the shoulder and elbow\ not only does the lost portion 

 of the humerus become formed afresh, but the radius and 

 ulna, and all the bones of the wrist and hand, are regenerated 

 accurately, even as regards the number of segments. It 

 seems hardly possible that so complex a structure could be 

 produced merely by the co-operation of proliferating cells, and 

 it might be supposed that an invisible power — a spiritus rector 

 or a vis forinativa — must be present to direct their mode of 

 increase and arrangement. We are nevertheless probably right 

 in assuming that no such external direction takes place, and 

 that the complex structures in living beings are produced merely 

 by the agency of the forces which are present in the individual 

 cells. 



We can understand these processes to some extent in the 

 case of embryogeny if we base our reasoning on the principle of 

 the gradual transformation of the idioplasm, which has already 

 been treated of in connection with ontogeny. This principle 

 may be roughly illustrated with respect to the skeleton of the 

 anterior extremity in the following manner. 



When the fore-limb of a Triton begins to arise as a small 

 blunt elevation of the skin, it consists of cells of the external 

 and middle embryonic layers. The whole of the former, and 

 that portion of the latter which forms the cutis, may be left out 

 of consideration ; they together give rise to the integument. 

 The rest of the mesoderm now forms a mass of cells which have 

 not yet begun to undergo differentiation, and which individually 

 do not apparently differ essentially from one another. They 

 must, nevertheless, be very different as regards the primary 

 constituents which they contain, for some of them w'ill subse- 

 quently give rise, for instance, to muscles, others to connective 

 tissue or to blood-vessels, and others, again, to bones. These 

 cells, which are so differently predisposed, must therefore con- 

 tain various determinants, which, when they obtain control over 

 them in the course of further cell-divisions, impress on the sub- 

 sequent generations the character of muscle- or bone-cells. 



