NOm2s |New GG, OF AMPA WAND STS CLEAVAGE. 349 
zigzag line, running irregularly, now to one side, now to the 
other, of the median plane. 
If homologies do not depend upon the form of cleavage, and 
may even unfold without any cleavage at all, how are we to 
explain the fact that in many cases (e.g., annelids, molluscs) 
cleavage is so constant, and so early becomes coincident with 
lines of homology? What determines this coincidence? This 
question has been discussed by one of us elsewhere, and we 
need not repeat here the conclusions reached. 
