OF LIVING FORMS ; OR, MORPHOLOGY. 99 



tration of the law of spiral growth. That organ 

 originates in a mass of pulsating cells, which, 



Fig. 20. 



Diagram of the circulation in a winged insect. The dark central portion represents 

 the heart ; it extends nearly the whole length of the body. 



gradually becoming hollow, gives the first form of 

 the heart in a straight tube, more or less subdivided, 

 and terminating at each extremity in blood-vessels. 



72 



