100 



DEVELOPMENT OF THE FROG'S EGG 



[Cn. X 



tion from a larger yolk-portion. It does not therefore divide 

 the egg, as in the preceding cases, symmetrically. 



Hertwig found that when eggs were compressed from above 

 downward, i.e. flattened axially between parallel plates, there 

 was no agreement between the plane of the first cleavage and 

 the median plane of the embryo. Four times the two coin- 

 cided, approximately, with the first furrow, five times with the 

 second, and six times with neither. The blastopore closes in 

 these eggs, as Born had also shown, at a point of the white 

 hemisphere opposite to that at which it first appeared. In the 

 eggs compressed from the sides and standing with the axis ver- 

 tical, the blastopore appeared generally at the edge between 

 the two plates, and closed at a point opposite to that at which 

 it had first appeared. 1 Exceptionally in these eggs the blasto- 

 pore appeared on one of the flattened surfaces, i.e. against one 

 of the compressing glass plates. 



EFFECT OF COMPRESSING THE EGG IN A GLASS TUBE 



Roux has shown that if the frog's egg be sucked up into a 

 glass tube of smaller diameter than the diameter of the egg, 



FIG. 32. Segmentation of egg enclosed in a tube. (After Hertwig.) A. Four-cell 

 B, C. Eight-cell stage, above and below. 



the egg will be drawn out into a barrel-shaped body and the 

 cleavage correspondingly modified. The results, however, are 

 not always alike. This is probably due to the presence of a 

 large amount of jelly surrounding the eggs, so that they do 



1 Hertwig found that when unsegmented eggs compressed between parallel 

 plates were rotated so that the white pole was turned upward, the egg rotated 



