BILATERALITY IN CRYPTOBRANCHUS 375 



At first, cell division proceeds most actively at the animal 

 pole; consequently we may regard this pole as the center of a 

 primary area of cellular activity. In stages 5 and 6 (illustrated 

 by figs. 16 and 17 of the present paper), also in the stages that 

 immediately follow, the frequent occurrence of a secondary area 

 of accelerated cell division has been noted (Smith, '12, II). On 

 opposite sides of the circular area occupied by the micromeres, 

 these cells are unequal in size and number; the smaller micro- 

 meres give evidence of more recent division, since the cleavage 



Figs. 16 and 17 Surface views of the upper hemispheres of two eggs of Crypto- 

 branchus allegheniensis in early blastula stages (stages 5 and 6, respectively), 

 showing excentric development of the micromeres. The figures were drawn with 

 the aid of a camera lucida. X 7. 



furrows that bound them are superficially deeper and more open, 

 as is always the case with newly formed furrows. In other words, 

 the region of most active cell division is no longer confined 

 to the vicinity of the animal pole, but extends from that pole a 

 short distance along a meridian. A line drawn from the center 

 of the secondary area of accelerated cell division through the 

 animal pole defines an axis of excentricity in the superficial 

 cleavage pattern of the micromeres. 



The condition is really one of bilateral symmetry, but to avoid 

 unwarranted implications I have tried to describe it without using 

 this term. The question naturally arises whether this excen- 



