8 EDWIN G. CONKLIN 
quent cleavages more nearly equal again, and yet it is certain that 
some such organization must be present. It is generally believed 
that the inequality of macromeres and micromeres is due to the 
quantity of yolk contained in the former and where the quantity 
of yolk is extremely great, as in Fulgur, this is undoubtedly one 
of the causes of the great difference in the sizes of the macromeres 
and micromeres; but that it is not the only cause of the inequality 
is shown by experiments in which by centrifuging eggs at the 
first or second cleavage two of the macromeres come to contain 
no yolk, while the other two contain all of the yolk; in the macro- 
meres which are purely protoplasmic and contain no yolk the 
subsequent cell divisions are still unequal, protoplasmic micro- 
meres of the usual size being separated from the protoplasmic 
macromeres, (see p. 81). The study of normal as well as of 
artificially altered cleavage points unmistakably to the conclu- 
sion that the position and axis of each spindle is fixed by the 
structure of the cell protoplasm, and since the position and axis 
of the spindles change regularly in successive divisions this 
protoplasmic structure must change regularly in successive cell 
generations. Boveri (’05) says that the position of the spindle 
is not due to a permanent cell structure, but that the constitution 
of the egg undergoes progressive alterations, which then react 
on the division centers. 
Among the micromeres certain cell divisions are quite unequal, 
and here there can be no question that this inequality of division 
is in no way associated with the presence of yolk, since the micro- 
meres are purely protoplasmic. In Crepidula the first and second 
subdivisions of the first quartet cells (figs. 3, 8), which give rise 
respectively to the ‘turret’ cells and the ‘apical rosette’ cells, 
are very unequal; as is also the division of the second quartet 
cells which give rise to the ‘tip’ cells of the arms of the ectodermal 
cross. The diameters of the two daughter cells in each of these 
divisions, and the approximate ratio of one to the other, are as 
follows in Crepidula plana: 
PSC Roe Oya iicylck lei eomcodom abacus ay pom nda acacH a: iat lO) cell 
Lal Git 3 0) dated 1Sp Ney ee eee Ratio5 :3 
PER ey, me (vec Warne Ronen ons a gsnananaedncndeaee Ratio 2:1 
