Brain PHysioLoagy oF Worms 349 
brain (Fig. 99) at the anterior end of a large (about 3 cm. 
long) Thysanozoon is cut off. The very small piece lying 
anteriorly then continues to creep or swim while the 
comparatively enormously large body executes no further 
progressive movements. The spontaneity of progressive 
movement in Thysanozoon is therefore 
a function of the brain. 
Both pieces of a divided Thysano- 
zoon continue to live and regenerate 
the missing portions. Only the oral 
piece regenerates more rapidly than the 
aboral, which has to form a head. I 
have not studied whether the latter 
forms a new brain. I kept such pieces 
alive for four months. The spontaneity 
of the aboral piece never returned, while 
the spontaneity of the oral piece persisted. 
2. The beheaded frog will remain upon its back when 
the cut lies behind the medulla oblongata; when the cut 
lies in front of the medulla the frog will not remain upon 
its back. We assume in this case that the geotropic func- 
tions of the ear compel the frog to resume the normal 
orientation. It is probable that the tactile stimuli also act 
in such a way that they compel the frog to bring the soles 
of its feet in contact with the surface of solid bodies, or to 
allow the weight of its body to press wpon the nerve endings 
in these portions of the skin. 
I designated the fact that an animal is compelled to orient 
its body in a definite way toward the surface of solid bodies 
as stereotropism. Geotropism cannot be demonstrated in 
Thysanozoon as the animal assumes any orientation toward 
the center of gravity for a long time. Stereotropism is, 
however, present as the animal is compelled to bring its 
ventral surface in contact with solid bodies, or to allow its 
FIG. 99 
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