8 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
cleavage, two theories have been proposed. According to the view 
which is perhaps that most generally known, the cause of unequal 
cleavage lies in the relative distribution of yolk material and formative 
protoplasm. The interaction between nucleus and cell contents, which 
determines the position of the dividing nucleus, exists only between the 
nucleus and the formative protoplasm, not between the nucleus and 
the yolk material. As a consequence of this interaction, the nucleus 
tends to take a position in the centre of the mass of Jormative proto- 
plasm. When one region of the cell is composed largely of yolk 
material, in a mere meshwork of protoplasm, while another region is 
made up entirely of protoplasm, the dividing nucleus must separate 
equal masses of formative protoplasm, and thus may divide the entire 
mass into very unequal parts, —one containing a certain mass of proto- 
plasm only, the other an equal mass of protoplasm and a large additional 
mass of yolk material. The theory has recently been formulated by 
Hertwig as follows : “Die Folge dieser Wechselwirkung aber ist, dass der 
Kern stets die Mitte seiner Wirkungssphüre einzunehmen sucht.. . . 
Wechselwirkungen finden zwischen dem Kern und dem Protoplasma, 
nicht aber zwischen ihm und dem Dottermaterial statt, wolches boi allen 
Theilungsprocessen sich wie eine passive Masse verhält. Ungleichmässig- 
keiten in den Protoplasmavertheiluug müssen sich daher auch auf Grund 
des obigen Satzes in der Lage des Kerns geltend machen, und zwar muss 
derselbe nach den Orten der grósseren Protoplasmaansammlung hin- 
rücken." (Hertwig, ’93, pp. 172 and 174.) 
Braem’s principle of equal resistance at both ends of the spindle is in 
character related to this view of Hertwig. Besides the effect of it in 
determining the direction of the spindle, this supposed principle is like- 
wise of effect in determining the equality or inequality of cleavage, as 
appears from the quotation from Braem given on page 7. 
C. Determination of the Time of Division, or the Interval between 
Successive Cleavages. 
The same factor which is held to determine the relative size of the 
cells was also held by Balfour, with whom Hertwig agrees (Hertwig, '93, 
p. 180), to determine the relative rapidity of cleavage. The greatest 
interval between successive cleavages is found in cells which contain the 
greatest amount of yolk relative to the amount of contained protoplasm. 
“The rapidity with which any part of an ovum segments varies ceteris 
paribus with the relative amount of protoplasm it contains; and the size 
of the segments formed varies inversely to the relative amount of the 
protoplasm.” (Balfour, '80, p. 99.) 
