92 ROSS G. HARRISON 
the yolk granules are packed closely into the small cell of the 
two-celled stage of Chaetopterus) seem to me to argue for a 
molecular basis of the fundamental principle of vital organi- 
zation." Morgan, likewise, takes this view when he says ('09, 
p. 114), "These considerations incline us to the view that there 
exist in the molecular constitution of the egg the potential 
factors of symmetry." The scheme outlined above is in harmonj- 
wdth this concept. 
On the other hand, Child ('13, '15), as also Delia Valle ('13) 
rejects all such hypotheses, basing the phenomena of axial differ- 
entiation upon the occurrence of gradients, which, according to 
Child, are primarily of a functional (metabolic) nature. It seems 
to the present writer that such gradients may well be an expres- 
sion of the polarity rather than its cause. 
G. Reduplication and the problem of polarity and heteromorphosis 
The reduplications which have been observed in the various 
experiments have already been described sufficiently for the pres- 
ent discussion (pp. 35, 45, 65, 73). The salient facts are: 1) that 
the duplicate is the mirror image of the original limb; 2) that 
more than one secondary member may arise by budding from the 
same primary bud, in which case both of the former stand in some 
relation of symmetry to the original; 3) that the secondary 
appendages themselves may be doubled, forming a more or less 
symmetrical pair. There are a few exceptional cases, where two 
members of the same side stand in linear series, but probably 
these have arisen only where the two rudiments are far enough 
apart not to influence one another.'"" 
^"^ Several cases are to be considered here. One (H. R. E. 10-) is a case in 
which the anterior half of the limb bud was removed. Two limbs developed, 
one clearly from the remaining posterior half and the other probably from the 
anterior border of the wound (cf. Harrison '18, p. 441). The operation was done 
on the left side and both limbs were lefts, the posterior one being somewhat 
defective. In another case, which had an early history similar to the above, the 
posterior member was very defective and it was impossible to determine whether 
it was a right or a left. A third case (H. R. E. 20) is the one figured on page 
79. This is not a case of regeneration, but one in which the anterior member 
probably developed from the grafted half, while the posterior member may have 
developed from the stationary ventral half. Both are attached to the same 
shoulder-girdle, but there are two separate glenoid cavities. 
