EFFECTS ON EGGS OF FRESH WATER PULMOXATES 447 
destined to give rise to certain organs, and that they are incapable 
of forming anything else. Apparently differentiation does not 
reach this stage before cleavage in these eggs. The differentia- 
tions of polarity and s>Tnmetr\' are, however, established before 
cleavage begins. But it is evident that polarity and symmetry do 
not reside in the clear substance alone from the fact that the dis- 
tribution of the clear substance with respect to the chief axis 
of the egg, or with respect to the plane of sA^mmetry may be ahnost 
as varied as in the case of the yellow and gray materials and j^et 
normal development may result. If there be a ground substance 
in the sense of Lillie (1906) in which polarity and bilaterality in- 
here, it should presumably be identified with the clear protoplasm 
and polarity and symmetry should be permanently altered when 
the mass of the clear substance is displaced. This is not always 
the case; most of the clear substance may go into one cell, the 
yellow into another and yet the final polarit}' and bilaterality of the 
developing embryo may not be changed. 
I cannot find therefore that either polarity or bilaterality is 
unalterably associated with anyone of the three visible substances 
of the egg. On the other hand it may possibly reside in some tenu- 
ous framework which*interpenetrates the entire cell and which is 
not disturbed by the shifting of the visible substances of the egg. 
I can offer no direct evidence for the existence of such a framework, 
distinct from the mass of hyaline protoplasm. Nevertheless it 
seems necessary- to assume that there is some permanent organ- 
ization which is not destroyed by centrifuging,and whichmay be 
able to bring back to their normal positions displaced nuclei and 
cytoplasmic substances. When most of the hyaline protoplasm 
goes into one of the first two blastomeres and most of the yolk 
into the other, such a return of displaced substances to their 
normal positions is impossible after the division wall between 
the two cells has been formed; but if there is a nucleus in the 
yolk cell, the cytoplasm of this cell may gradually increase in 
amount until approximately normal relations are restored. While 
therefore it is possible for the visible polarity or bilateralit}^ of 
an egg to be changed experimentally, there is apparently some- 
thing which does not change, and which is able to restore, by a 
process of regulation, the original organization. 
