No. 3. ] THE CLEAVAGE OF AMPHIBIAN OVA. 413 
20, 28, for example, show that cells originally to one side of the 
mid-line have been so shifted by the stresses of cell-division 
as to lie unmistakably on the opposite side. Everything indi- 
cates that the extent of this shifting increases greatly in later 
stages. It is, of course, open to any one to suppose that the 
cells thus driven out of their original position regain this posi- 
tion later on. The probabilities against such an occurrence, 
however, seem to us so great that we feel justified in considering 
that the burden of proof rests on those who would maintain that 
this readjustment takes place. It seems to us a more reason- 
able supposition that the direction of the early cleavage planes 
and the embryonic axes have no vital connection, and that the 
coincidence, where it exists, is in itself of no fundamental 
significance, 
It seems to be unnecessary to dwell here upon the question 
of cell homologies. Reference to our figures will show abun- 
dantly that the derivatives of equivalent quadrants do not hold 
similar positions. There exists in this respect a fundamental 
difference between these vertebrates and certain invertebrate 
groups, notably the annelids. In the amphibia, the more de- 
tailed the study, the more frequent and radical appear the 
cleavage variations, while in the annelids, as Wilson (92) ex- 
presses it, “the cleavage of the ovum takes place with a 
precision and regularity which oft-repeated study only renders 
more striking and wonderful.” 
THE UNIVERSITY OF CHICAGO, 
March, 1893. 
