DISUSE AND ITS EFFECTS. 39 
membrane occupying the space known as the pupil of 
the eye. This membrane rapidly atrophies and the 
pups begin to see. In the human offspring a precisely 
similar membrane is present during em- 
bryonic life, but atrophies a few weeks 
before the termination of intra-uterine life, 
The mode by which foetal or larval organs 
of this character are slowly removed will 
be considered in detail in a later chapter. 
Suppression of Parts.—It has long been 
known that in the embryo of vertebrates 
OUTH 
TAIL SPIRACLE ae a 
t i / 
ASCIDIAN 
NOTOCHORD 
Fic, 1g9.—Ascidian Tadpoles (after Lankester), A, An 
adult ascidian. 
many structures and organs are formed and then dis- 
appear without attaining, as far as our knowledge allows 
us to judge, a functional condition. 
