EMBRYOLOGY OF CRYPTOBRANCHUS 485 



furche' or 'septal furrow') bounding the posterior margin of the 

 roof of the segmentation cavity, parallel to the future blastopore. 

 Such a furrow does not normally occur in this stage of Crypto- 

 branchus allegheniensis, but a similar furrow makes its appear- 

 ance shortly after the beginning of gastrulation. 



B. Summary 



The cleavage is holoblastic, but with great inequality in the 

 size of the micromeres as compared with the macromeres. 



Biradial symmetry of the cleavage pattern begins with the 

 third cleavage stage. In the upper hemisphere, as a consequence 

 of the shifting of the micromeres, this biradial symmetry is lost 

 at about the fifth cleavage stage. In the lower hemisphere, 

 because of the slow cleavage and stability of the macromeres, 

 it persists until after the beginning of gastrulation, and in some 

 eggs enables one to distinguish first and second cleavage furrows 

 even after the blastopore has appeared. 



An excentrically situated area of unusually small micromeres 

 is apparent in surface views of most eggs in Stages 5 to 8 inclu- 

 sive; the cleavage pattern of such eggs thus possesses bilateral 

 symmetry. 



In the sixth cleavage stage there is more rapid cell division 

 in the marginal region of the micromeres than in the region of 

 the animal pole. The later expression of this tendency is almost 

 wholly internal. 



In Stages 7 and 8 some of the superficially smaller micromeres 

 are becoming submerged through a process of immigration. 



In the later cleavage stages there is a tendency for the cleavage 

 furrows to become less distinct than formerly in the region of 

 the vegetal pole, indicating a difficulty in sustaining the holo- 

 blastic character of the cleavage in an egg so heavily laden with 

 yolk; the same tendency is observed in Necturus. 



In late segmentation immediately preceding gastrulation the 

 cleavage pattern enables one to predict the side on which the 

 blastopore is to appear; the egg undergoes a slight rotation, on 

 a horizontal axis at right angles to the median plane. 



