348 WHITMAN AND EVYCLESHVMER. [Vor XIE 
ganoids? How are we to determine homology in such a case ? 
To suppose, as some authors have done, that the first horizontal 
cleavage of the frog’s egg is skipped in the case of the fish egg, 
is nonsense. There is no skipping in cleavage so long as the 
division of the cytoplasm follows regularly the order of nuclear 
division. If nuclear division runs on unaccompanied by seg- 
mentation of the egg, as in the insect egg, then we may speak 
of skipping cleavage; if cleavage does not appear until after 
three or more nuclear divisions, and then begins and takes the 
regular course, splitting into two, four, eight, etc., as in the 
crab’s egg, according to Paul Meyer, we may speak of deferred 
cleavage. In the fish egg the cleavage is neither deferred nor 
skipped. Where, then, is the homologue of the third cleavage 
of the frog’s egg? If position and relation to the axes of the 
future embryo are to decide homology, then there is no homo- 
logue before the fifth cleavage, and no certainty of any even 
then. To assume homology between a cleavage of the 4-cell 
stage and one of the 16-cell stage is to venture into a hopeless 
tangle of anachronisms. If order of succession is to be our 
criterion, then first is first, second is second, third is third, and 
so on, without regard to the position of the planes of cleavage 
or the fate of the cells. In either case, the attempt to homolo- 
gize cleavages leads to contradictions and confusion. Homol- 
ogy that means nothing beyond the second cleavage, or that 
may fail even at the first cleavage, is its own negation. The 
wide range of variation in cleavage in many eggs, its total sup- 
pression in many others, and the possibility of changing its 
course by artificial means, without affecting the final result, all 
go to show, as it seems to us, that homologies do not depend 
upon cleavage planes. Even the first and second cleavages, 
which appear, at first sight, to agree so closely in widely differ- 
ent eggs, may cross the future lines of homology at various 
angles. If there is any homology in cleavages worth talking 
about, it must be because they take definite and constant parts 
in defining homologous organs or areas. Something of this 
sort has been claimed for the first cleavage. On further exam- 
ination the claim proves to be ill founded. Often, as early as 
the 8 and 16-cell stages, the first groove is transformed into a 

