8 bulletin: museum of comparative zoology. 



cleavage, two theories have been proposed. According to the view 

 ivhicli 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 formative 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- 

 phism 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 "Wirkungssphare einzunehmen sucht. . . . 

 Wechselwirkuugen finden zwischen dem Kern und dem Protoplasma, 

 nicht aber zwischen ihm und dem Dotterraaterial statt, welches bei alien 

 Theilungsprocessen sich wie eine passive Masse verhalt. Ungleichmassig- 

 keiten in den Protoplasmavertheiluug miissen sich daher audi auf Grund 

 des obigen Satzes in der Lage des Kerns geltend machen, und zwar muss 

 derseliie nach den Orten der grosseren Protoplasmaausammlung hin- 

 ruckeii." (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.) 



