METAMORPHOSIS. ' 55 



entire reconstruction of the larva. The reconstructive 

 stage is known as z. pupa (or pupal stage), and the whole 

 change is termed a ?nefamorp hosts. 



The insects show us a complete series in the origin of 

 metamorphosis. One instance, of the silkworm moth, must 

 suffice : the caterpillar, or silkworm, is a worm-like larva 

 which lives often for a considerable time with all the 

 functions active except that of reproduction. It then 

 becomes transformed into a quiescent pupa and a number 

 of its organs are broken down and others constructed until, 

 finally, the perfect winged moth is set free. 



Some instances of this divergent evolution of two stages in the life of 

 one individual have a deceptive likeness to the growth of a fresh indi- 

 vidual or generation upon the preceding one. 



Summary. — 



The individual commences life as 2, fertilised unicellular ovum. 



By growth, cell-division and differentiation, it is converted into the 

 adult organism. 



The early cell-division is called segmentation^ which varies in type 

 according to the quantity and arrangement of the yolk. 



Segmentation usually results in the production of a monoblastic 

 stage, with one primary layer, or archiblast. 



The archiblast is converted into two primary layers, the epiblast 

 and hypoblast^ forming a diploblastic stage. 



The adult may remain at this stage or the third primary layer, 

 mesoblast, may be produced, forming a triploblastic organism. 



Each primary layer then produces a series of organs in regular 

 sequence. 



The primary layers and the other organs all arise by one of three 

 methods. 



All or part of a development may be larval or embiyonic. 



In larval development, a divergent evolution of larva and adult 

 produces a metamorphosis. 



