EMBRYOLOGY OF CRYPTOBRANCHUS 483 



the micromeres and transitional cells approach nearer the vegetal 

 pole. A meridian drawn through the vegetal pole and the center 

 of the area occupied by the macromeres defines the axis of excen- 

 tricity; this axis bears no constant relation to the first cleavage 

 furrow and the axes of biradial symmetry determined by the 

 early cleavage furrows. The biradial symmetry of the cleavage 

 pattern is of course somewhat disguised by the more rapid multi- 

 plication of cells at one end of the axis of excentricity. 



In this stage occurs a slight tilting of the morphological axis 

 of the egg within a meridional plane determined by the axis of 

 excentricity, so that the vegetal pole no longer coincides with 

 the lower pole as determined by gravity. The vegetal pole is 

 shghtly uptilted on the side where the more rapid multiplication 

 of cells occurs, hence the meridian defining the axis of excentricity 

 passes also through the new pole determined by gravity. This 

 new pole at first lies intermediate between the vegetal pole and 

 the center of the area occupied by the macromeres; in later 

 stages, through continued tilting of the egg in the same direction, 

 it comes to lie beyond this center. Throughout the ensuing 

 stages we must distinguish between the morphological axis of 

 the egg as determined by the animal and vegetal poles, and the 

 vertical axis determined by gravity. The method of locating 

 the vertical axis, and exact measurement of the amount of rota- 

 tion, will be given in the following stage. 



If the egg be sectioned along the axis of excentricity the internal 

 structure, to be described later, shows that this axis lies in the 

 sagittal plane of the embryo ; the side on which the small cells 

 approach nearer to the vegetal pole is the one on which the blasto- 

 pore is to appear. Thus the excentric position of the vegetal 

 pole within the area occupied by the macromeres enables one to 

 orient the egg with reference to future body regions. 



In perhaps the majority of cases, the transition from large 

 to small cells is more evenly graded on the side where it occurs 

 nearest to the vegetal pole, than on the opposite side where it 

 is characterized by a rather abrupt line of demarcation (figs. 

 Ill and 112). This feature, when present, gives a true bilateral 

 symmetry to the cleavage pattern of the lower hemisphere; the 

 axis of this bilateral symmetry coincides with the axis of excen- 



