238 



GENERAL EMBRYOLOGY 



A rather special form of bilateral cleavage known as the 

 disymmetrical type is found in the Ctenophores. In these 

 Ccelenterates the first and second furrows are meridional, the 

 planes are complete, and divide the egg adequally. The 

 third, a double cleavage, is oblique to the first, passing in the 

 same general direction as this, on each side of it, but approach- 



FIG. 114. Diagrammatic representation of the cleavage in the Ctenophore 

 (based upon Beroe). After Ziegler. A. Four-cells, from side and above. 

 B. Eight-cells in side view (the animal pole is downward here, and in D). C. 

 Eight-cells, from animal pole. D. Sixteen-cells, from side. E. Sixteen-cells, 

 from animal pole, ra, ra. Median plane; t, t, transverse plane. 



ing the vegetal pole more closely than the animal. The descend- 

 ants of each of the first two cells then become symmetrically 

 arranged about the second plane so that two similar groups of 

 cells are formed, each group bilaterally symmetrical with refer- 

 ence to a plane perpendicular to the plane of symmetry of the 

 entire cell group (Fig. 114). 



