482 



BERTRAM G. SMITH 



to the naked eye, and barely distinguishable with the magnifi- 

 cation used for photographing ( X 4) . The area occupied by 

 the micromeres extends approximately to the equator, though 

 the broad zone of transition makes it difficult to define. In the 

 vicinity of the vegetal pole, the cleavage furrows have again 

 become faint; in many cases, in preserved material, they are 

 distinguishable as fine lines lighter in color than the general sur- 

 face, rather than as actual grooves. For the accurate study of 

 these furrows in this and the following stage, a binocular micro- 

 scope is usually required. When their boundaries are distinct, 

 on account of their large size the macromeres are readily seen 

 with the naked eye. 



Figs. Ill and 112 Lower hemispheres of two eggs of Cryptobranchus alleghe- 

 niensis in Stage 10, showing cleavage pattern. The embryo shown in figure 112 

 is slightly older than the one represented in figure HI. Camera drawings from 

 preserved material. In each egg, the lower pole as determined by gravity lies 

 at the center of the figure; the vegetal pole, at the intersection of the first two 

 cleavage furrows, is slightly above this point. The upper part of each figure rep- 

 resents the side on which the blastopore is to appear. X 7. 



Usually, the cleavage pattern of the lower hemisphere retains 

 enough of its earlier bilateral symmetry to enable one to distin- 

 guish first and second cleavage furrows. The vegetal pole, since 

 it occurs at the intersection of the first two cleavage furrows, 

 may in most cases still be determined quite accurately and con- 

 veniently by means of the cleavage pattern. As shown in figures 

 111 and 112, the vegetal pole is excentrically located in the area 

 occupied by the macromeres; a more rapid multiplication of 

 cells has occurred on one side of this area, so that on this side 



