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CLEAVAGE (SEGMENTATION) AND BLASTULATION 



Fig. 151. Cleavage in the egg of Necturus macidosus. (After Eycleshymer and Wilson, 

 '10.) (A) First two cleavage planes are meridional. (B) Third cleavage planes tend 

 to be vertical and meridional. (C) Fourth cleavage planes are vertical, meridional, and 

 irregular. (D H) Following cleavage planes become irregular, offering a mixture of 

 modified latitudinal, vertical, and meridional varieties. 



third cleavage furrows are irregularly vertical (fig. 15 IB), while the fourth are 

 latitudinal, cutting off four very irregular micromeres at the animal pole. Seg- 

 mentation then becomes exceedingly irregular. (See Eycleshymer and Wilson, 

 '10). One characteristic of cleavage in Necturus is a torsion and twisting of 

 the cleavage grooves due to a shifting in the position of the blastomeres. 



As shown in the figures, the first three cleavage planes assume a distinct 

 meroblastic pattern of two meridional furrows followed by vertical furrows. 

 The yolk material evidently impedes the progress of the furrows considerably. 



4) Acipenser sturio. In the genus Acipenser are placed the cartilaginous 

 ganoid fishes. Cleavage in Acipenser sturio, the sturgeon, resembles that of 



