100 LIFE IN NATURE. 



This is the permanent form of the heart in many 

 animals. Fig. 20 represents the heart of an insect. 

 When the organ is to be developed into a more com- 

 plex form, the first step in the process is its twisting 

 into the shape shown in Fig. 21. 

 It is like what takes place when 

 we hold a flexible rod in our hands, 

 and gradually approximate its ends. 

 The straight tube is growing within 

 a limited space, and therefore " coils 



Bischoff). The central . 



expanded portion is this fundamental form it retains 



the heart ; above and 



vessels communicat- throughout all its subsequent de- 



ing with it. 



velopment. 



But if this principle is true, why has it been 

 overlooked? and why have men fallen into a way 

 of speaking as if living matter had some inherent 

 tendency to grow into certain forms, or as if masses 

 of cells could model themselves, by some faculty 

 or power of their own, into elaborate and complex 

 shapes ? 



It seems a strange thing that they should have 

 done so, and yet it may easily be accounted for. 



