DEVELOPMENT. 215 
velop into very different forms. Yet a symmetry of plan 
underlies the structure of all animals. In the embryo, 
this symmetry of the two ends, as well as the two sides, 
is nearly perfect; but it is subsequently interfered with to 
adapt the animal to its special conditions of life. It is a 
law that an animal grows equally in those directions in 
which the incident forces are equal. The Crinoid, rooted 
to the rocks, is subjected to like conditions on all sides, 
and, therefore, it has no right and left, or fore and hind 
parts. The lower forms, generally, are more or less geo- 
metrical figures: spheroidal, as the Sea-urchin ; radiate, 
as the Star-fish; and spiral, as many Foraminifers. The 
higher animals are subjected to a greater variety of con- 
ditions. Thus, a Fish, always going through the water 
head foremost, must show considerable difference between 
the head and the hinder end; er a ‘Lurtle, moving over 
the ground with the same surface always down, must have 
distinct dorsal and ventral sides. 
Nevertheless, there is a striking likeness between thre 
two halves or auy two organs situated on opposite sides 
of an axis. And, first, a belateral symmetry is most con- 
stant. It is best exhibited by the Articulates and Verte- 
brates; but nearly all animals can be clearly divided into 
right and left sides—in other words, they appear to be 
double. A vertical plane would divide into two equal 
parts our brain, spinal cord, vertebral column, organs of 
sight, hearing, and smell; our teeth, jaws, limbs, lungs, 
etc. In fact, the two haives of every egg are identical. 
There are many exceptions: the heart and liver of the 
higher Vertebrates are eccentric; the nervous system of 
Mollusks is scattered; the hemispheres of the human 
brain are sometimes unequal; the corresponding bones in 
the right and left arms are not precisely the same length 
and weight; the Narwhal has an immense tusk on the left 
side, with none to speak of on the other: Rabbits have 
