DEVELOPMENT. 
207 
organs. For a time, indeed, the human embryo so closely 
resembles that of the lower forms as to be indistinguisha¬ 
ble from them; but certain structures belonging to those 
forms are kept long after the embryo is clearly human. 115 
All the members of a group do not reach the same degree 
of perfection, some remaining in what corresponds to the 
immature stages of the higher animals. Such may be 
called permanently embryonic forms. 
Sometimes an embryo develops an organ in a rudimen¬ 
tary condition, which is lost or useless in the adult. Thus, 
the Greenland Whale, when grown up, has not a tooth in 
its head, while in the embryo life it has teeth in both 
jaws; unborn Calves have canines and upper incisors; 
and the female Dugong has tusks which never cut the 
gum. The “splint-bones” in the Horse’s foot are unfin¬ 
ished metatarsals. 
Animals differ widely in the degree of development 
reached at ovulation and at birth. The eggs of Frogs 
are laid when they can hardly be said to have become 
fully formed as eggs. The eggs of Birds are laid when 
segmentation is complete, while the eggs of Mammals are 
retained by the parent till after the egg-stage is passed. 118 
Ruminants and terrestrial Birds are born wfith the power 
of sight and locomotion. Most Carnivores, Rodents, and 
perching Birds come into the world blind and helpless; 
while the human infant is dependent for a much longer 
time. 
1. Metamorphosis. 
Few animals come forth from the egg in perfect condi¬ 
tion. The vast majority pass through a great variety of 
forms before reaching maturity. These metamorphoses 
(which are merely periods of growth) are not peculiar to 
Insects, though more apparent in them. Man himself is 
developed on the same general principles as the Butterfly, 
but the transformations are concealed from view. The 
