1044 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 
plate cells, А?* and А?*, while the lower yolk-laden part becomes the chorda cells, 
A“ and A‘. The chorda-neural substances are thus contained in the same cells 
until the sixth cleavage, though their substances are distinct at a much earlier 
period. Still other instances might be cited to show that the planes of local- 
ization and the planes of cleavage do not always coincide. Thisis in part due to the 
fact that the boundaries of the different kinds of germinal material, e. g., the yellow 
protoplasm of the Cyxthza egg, are not as sharp as are the boundaries of the cells, 
and consequently the cleavage furrows cannot precisely separate different kinds of 
germinal material. Nevertheless the cleavage planes are, under normal conditions, 
constant in position and character and bear a constant relation to the planes of dif- 
ferentiation. But that this relationship is not a casual one is further indicated by 
experimental studies on cleavage in which the position of the cleavage furrows may 
be altered without altering the localization of germinal materials or the typical form 
of development. Therefore the factors which determine localization and those 
which determine the form of cleavage are more or less independent. 
АП of these facts speak unmistakably for the view that localization is more 
fundamental than cleavage as Whitman (1895) has so ably maintained, and that 
such correspondence as may exist between the two 1з of secondary origin and of 
minor importance. Nevertheless the extreme constancy of cleavage forms shows 
that we have here a phenomenon, which if of secondary importance to germinal 
localization, is still of real significance. I have shown that in Crepidula the 
cleavage is a localizing factor, though secondary in importance to protoplasmic 
movement, and it seems probable that Wilson (1903) is right when he argues that 
the relative isolation produced by cleavage gives opportunity for the increase of 
any initial differences which may exist in the cells at the time of their formation. 
Finally it must be concluded as a result of both observation and experiment 
that the type of cleavage is less constant and less fundamental than the type of 
localization, but that cleavage may itself be a factor in the progressive specification 
of cells (cf. Wilson, Lillie, Conklin, eż а2.). 
E. TYPES оғ GERMINAL LOCALIZATION; EVOLUTION or TYPES. 
The wonderful resemblances in the germinal localization of annelids and 
mollusks, as shown especially in the cleavage, have been repeatedly commented 
upon. Furthermore this localization is for shadowed in the egg before cleavage 
begins, and this suggests the inquiry as to whether the resemblances between types 
of localization grow closer as one approaches the ovocyte, and whether the man- 
ner as well as the results of localization are comparable in the different types. At 
present our knowledge of the localization in these earliest stages of development 
is very incomplete, and a comparison can be drawn only between annelids, mol- 
lusks, ctenophores, echinoderms, ascidians and possibly nemerteans and nematodes. 
In most of these phyla a peripheral layer of protoplasm is present before 
maturation, which after maturation and fertilization collects at one or both poles 
of the egg; also with the possible exception of the ctenophores and nematodes, 
