KOFOID. 



LAWS OF CLEAVAGE. 



185 



spiral period of cleavage may therefore be designated by three 

 characters: (1) a letter, as a, b, c, d, indicating the quadrant ; 

 (2) a first exponent indicating the generation, as a*', a 5 -, etc. ; (3) a 

 second exponent indicating the quartet or story, as a 4 - 1 , a 4,2 , a 7,10 , 

 etc. Generations are numbered starting with the ovum as the 

 first generation. The number of cells doubles with each succeed- 

 ing generation, and after the third generation, i. e. after the 

 four-cell stage or the first quartet, the number of quartets is also 

 doubled. The quartets or stories are numbered from the vegeta- 

 tive toward the animal pole. Thus in the eight-cell stage the 

 lower quartet is designated by the exponent 1, and the upper 

 quartet by the exponent 2. This principle is followed in the 

 nomenclature of all quartets. During the period of spiral cleav- 

 age, the two daughter cells (or quartets) resulting from the 

 cleavage of any given cell (or quartet) never lie in the same plane, 

 and the lower cell (or quartet) of the two is always designated 

 by an odd exponent and the upper by an even one. Thus, when 

 ft 5.3 ^ — ^5.3) di v i c i eSj the resulting cells (quartets) are a. 6 - 5 ( — d 6 - 5 ) 

 and a 6,6 ( — <Z 6,6 ), the latter being nearer the animal pole. The 

 second exponent of the upper daughter cell is always twice the 

 corresponding exponent of the mother cell, and that of the lower 

 cell twice less one. The quartets are thus designated as though 

 all the quartets of their generation were actually present, a con- 

 dition rarely realized in later generations ; however, these quar- 

 tets are potentially present, being represented by their ancestors 

 or descendants, and no confusion need arise over this point. 



The following is a scheme of the nomenclature through the 

 sixth generation : — 



