234 Herbert W. Rand 
earthworm and planarians, where a head-like growth may take 
place at the posterior surface of a very short anterior piece of the 
worm or a tail-like growth may appear at the anterior surface of a 
short posterior piece. Just as the capacity for regenerating a 
head in normal orientation is most strongly present in the extreme 
anterior region of the worm, so in the actinian the nipple is formed 
most promptly and constantly at distal cut surfaces near the tip 
of the tentacle. In both cases there is doubtless a connection 
between the fact that a certain capacity is concentrated in a par- 
ticular region of the organism, and the fact that this capacity 
occasionally expresses itself there in an orientation which 1s the 
reverse of the usual one. 
The morphological polarity seen in the tentacle becomes most 
striking when we consider the fact that every region of the ten- 
tacle. (possibly excepting the extreme tip) possesses the capacity 
of behaving in two quite different ways. No matter at what level 
the tentacle is cut, a proximal cut end always shows one-type of 
behavior, a distal cut end another. Assuming arbitrarily, for con- 
venience, that the activities which characterize the behavior of a 
distal cut end extend over five millimeters of the length of the ten- 
tacle, then we may say that any transverse zone or band of the 
tentacle, of such length, is capable of these two types of behaviour. 
When a tentacle is cut so that a certain one of these arbitrarily 
distinguished zones happens to lie at the distal end of a fragment, 
the behavior peculiar to a distal end is called forth. . But if that 
same zone of tissue had happened to lie at the proximal end of a 
fragment, then it would have responded in the very different way 
which characterizes a proximal end. It readily appears that the 
position of any given zone of tissue in relation to a cut surface is the 
occasion which determines whether that tissue shall behave one 
way or the other. But when we inquire what is the means by 
which always the appropriate one of these two alternative re- 
sponses is called forth, we soon find ourselves at the limit of our 
present knowledge. However, some suggestions may be made. 
The distinctive feature of the response of the distal cut end is 
contraction. In all probability this is a contraction of more or 
less specialized muscle elements—the circular muscle-processes 
