IT. i CELL-DIVISION 53 



the cells of a radially segmenting egg, such as that of Rana 

 fusca, are governed by the same influences as determine the 

 pattern of the drops. 



The resemblances, it will be conceded, are often very close. 

 There are also important differences. The polar furrow, which 

 is often present in the Frog's egg, is not necessarily between the 

 cells with the greatest mass. Again if, in the four-celled stage, 

 with no polar furrow, one of the cells be diminished by puncture, 

 a polar furrow does not always appear, as it would with oil-drops, 

 nor, if it does, is it always formed by the union of the larger 

 cells. Or, if when a polar furrow is present between the larger 

 cells, one of these is diminished by puncture until it, together 

 with its opposite, is less than the other two, the polar furrow 

 nevertheless retains its position. 



In the sixteen-celled stage the animal cells together form a 

 ring of eight around the axis. The cells are not necessarily equal, 

 and a small cell may be compressed by, instead of compressing, 

 adjacent large ones, while they, not it, move away to the periphery. 



Other differences are that large cells bulge into small, that 

 cells are elongated tangentially instead of radially, that there 

 are amoeboid processes at the inner ends of the cells, and inter- 

 cellular spaces between them. 



Further, Roux has examined the behaviour of the isolated 

 cells of the Frog's egg in the morula stage. The cells were 

 separated in a medium of albumen, or salt-solution, or a mixture 

 of the two. They first approach and then flatten against one 

 another (Cytarme), as do the blastomeres in the egg, completely 

 or incompletely. The contact surface is generally symmetrical 

 to the line joining the centres of mass of the two cells ; it may be 

 concave towards either the small or the large cell. More than 

 two cells may unite to form rows or heaps. The angles made by 

 the surfaces of contact may be 120, or have other values. Four 

 surfaces may meet in one line ; at other times the arrangement 

 is tetrahedral. In a 1-25 % solution of salt the cells are 

 elongated, and united end to end in long branching strings. 

 The pigment, diffused through the cell, later returns to its 

 original position at the surface, or usually to the middle of the 

 free surface of each. 



The cells may also move over one another (Cytolisthesis) by 



