54 



ME TA MORPHOSIS. 



in the animal kingdom. In a very general way, the organs 

 usually arise from the three primary layers as follows : — 



1. Epiblast. — Epidermis. 



Sense-organs and nervous system. 

 Excretory system (partly mesoblast). 



2. Mesoblast. — Muscular system. 



Skeletal system. 

 Blood-vascular system. 

 Reproductive system. 



3. Hypoblast. — Alimentary system. 



Fig. 21.— The Metamorphosis of the Silk- worm Moth. 



The larva or caterpillar spins a cocoon and changes into a pupa (on the stem) 

 which later gives rise to the winged moth. Both sexes are shown. 



Metamorphosis. — From the foregoing we see that 

 in the course of time any stage in the life -history of a species 

 may, to meet a special environment, be especially arid coeno- 

 genetically developed until a larval stage is produced in 

 marked contrast to the adult. 



In some instances this independent evolution of two 

 stages in the life of one organism has reached such a 

 climax that the adult stage can only be reached by an 



