METAMORPHOSIS. 55 
entire reconstruction of the larva. The reconstructive 
stage is known as a pupa (or pupal stage), and the whole 
change is termed a metamorphosis. 
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 avergent 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 a 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 archdblast. 
The archiblast is converted into two primary layers, the epzblast 
and hyfodblast, forming a diplodlastic 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 ovgaxs 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 /arval or embryonzc. 
In larval development, a divergent evolution of larva and adult 
produces a metamorphosis. 
