CH. X] MODIFICATION OF CLEAVAGE 101 



not always take the shape of the enclosing tube. In order to 

 avoid this inconstant element, Hertwig ('93) repeated the 

 experiment with eggs from which much of the jelly had been 

 cut away. The fertilized eggs were drawn up into cylin- 

 drical tubes in which they assumed a short cylindrical shape 

 (Fig. 32). The eggs lay with the black hemisphere against 

 one side of the tube and this side was turned upward, and the 

 tube kept in a horizontal position. The first cleavage of such 

 an egg is vertical and at right angles to the long axis of the 

 tube (Fig. 32). The second furrows are also vertical and at 

 right angles to the first, therefore in the direction of the long 

 axis of the tube. -The third furrows are also vertical and 

 parallel to the first. The result so far is the same as when the 

 eggs are compressed from above downward between parallel 

 glass plates. The fourth furrows are horizontal and divide the 

 egg into eight black and eight white cells. 



CONCLUSIONS FROM THE EXPERIMENTS 



These experiments in which the cleavage has been modified 

 by changing the shape of the egg have an important bearing 

 on the general problem of cleavage of the egg. In the first 

 place, the " induced " form of the cleavage may give us some 

 insight into the causes that determine the direction of the 

 normal cleavage-furrows. In the second place, we see that 

 when an egg is compressed, the sequence of the cell-division is 

 very different from the normal sequence. Since we get nor- 

 mal embryos from eggs modified in this way, it would seem to 

 follow, as Pfliiger was the first to point out, that the cleavage 

 simply divides the spherical egg into the building-blocks from 

 which the later embryo forms, and it is a matter of indifference 



as a whole and tended to turn the white hemisphere downward. If, however, 

 the eggs were compressed after the two, four, or eight cell stage, they then held 

 their position much better when the white pole was turned upward. If the 

 compression was applied when the cleavage of the eggs had gone very far, but 

 before the blastopore appeared, it was again found that the rotation of the egg 

 as a whole takes place (as in the unsegmented egg). An egg that has been 

 turned with its white pole upward at the two or four cell stage and has kept its 

 position during the cleavage-period, no longer tends to rotate as a whole during 

 the later stages of cleavage. 



