174 GEOLOGICAL BIOLOGY. 



(up to the formation of the completed organs — those, at least, 

 that are essential to independent activity) is carried on with 

 relative passivity of the embryo itself; and the determination 

 of all this development is traceable directly to the parent, and 

 not to the environment of the developing organism. How- 

 ever much the length and extent of this embryo stage may 

 differ in different kinds of animals, it is clear that there is 

 such an embryo stage of development in all metazoa. 



The Stage from the Free Existence of the Individual to the 

 Maturing of its Functions, — Sixth. The next step in the de- 

 velopment is the setting free of the organism from its 

 embryo stage ; its birth marks the beginning of the infantine 

 stage in the higher Vertebrates. The higher the differentia- 

 tion and the more complex and specialized the organization, 

 so much the longer is the dependent or preparatory stage 

 extended. 



In the higher animals, for instance, some of the systems 

 of organs are not completed at birth, particularly the genera- 

 tive system ; these gradually mature, and the stage from 

 birth to the perfection of this system of organs is the infantine, 

 larval, or juvenescent stage. Full maturity is reached only 

 when the whole organism is fully developed and capable of 

 independent life and the execution of all the functions of life. 



The Cell an Organism. — From what has already been said 

 the essential elements of the organism may be learned. Re- 

 curring to Kant's definition of an organism as "That whose 

 every part is at once the means and end of the whole," we 

 observe that one of the first marks by which we recognize the 

 simplest cell to be an organism is its division into parts, with 

 what we assume to be different functions, because they do 

 play different parts in the history of the cell. 



Differentiation of the Cell a Mark of its Organic Nature. — 

 The simplest differentiation of parts which we are able to 

 observe is that expressed by the cell-ivall. This is a differentia- 

 tion of the superficies as a protective shelter for the interior. 

 If, in contrast, we break open a crystal there is no essential dif- 

 ference between the outer and inner parts. A further special- 

 ization of parts and function is seen in the nucleus as a differ- 

 entiated part of the cell. All cells do not appear to be pos- 



