480 OSCAR RIDDLE 
tion. There has been no theory to cover the long series of points 
involved, some of which are the following: What are the con- 
ditions which permit yolk to form? In what situations and about 
what structures does it form (this point much studied and dis- 
cussed)? What are the processes involved,—what is the mech- 
anism of yolk formation? How are the different forms of yolk 
genetically and chemically related? How account for the vari- 
able amount, distribution, and stratification of yolk? 
The statements concerning cell-organs as directive agents of 
yolk-building have often been quite misleading. This could 
hardly be otherwise since we have had here attempts to ‘explain’ 
a process, not in terms of other processes, but in terms of struc- 
twre—an error not uncommon even in modern biology. One gets 
the idea from some descriptions of yolk formation that the nucleus 
is the absolute, immediate and ultimate source of yolk; and this 
in spite of the fact that yolk is never present within the nucleus, 
but only outside of it. Just how a vanishingly small fragment of 
chromatin, thrust from nucleus into cytoplasm—7.e., into an 
environment so new as to imperil its own existence,—may guide 
and direct the very rapid production of a thousand times its own 
volume of yolk (a new and very different substance from itself) 
we have not been told. Much apparently has been left to the 
imagination of the reader who is evidently expected to bridge for 
himself the gap that exists between the chromatin particle in situ 
and the yolk building process in operatio. But, the high regard © 
which some adherents of this theory have for the kingly chromatin 
evidently persuades them that chromatin particles—which cer- 
tainly are thrown from the nucleus into the cytoplasm, and about 
which traces of yolk certainly are sometimes found—comprise 
material of such superior quality that the base and foreign matter 
which meets their Midas-like touch must turn at once into golden 
yolk! By other workers mitochondria, and still other structures, 
have been similarly endowed with what would seem to be wonder- 
ful and transforming power. The writer would not undervalue 
the great amount of very valuable work that has led to the deter- 
mination of the cell-elements about which yolk forms. But it 
seems to him that much less valuable than this painstaking work 
