188 PROCEEDINGS OF THE AMERICAN ACADEMY. 



reference to the vertical axis, i. e. the direction of their obliquity 

 is the same, so that if they should be spread out in a plane paral- 

 lel with the vertical axis of the egg they would be approximately 

 parallel to one another. The left aster is in every case the upper 

 one ; and at the end of division the upper derivative will lie to 

 the left of the lower one, not to the right, as in the preceding 

 and in the following generation. The division of the cells will 

 take place in a plane approximately at right angles to that of the 

 preceding division, and approximately parallel to the one preced- 

 ing that. This alternation of the direction of the spindles and 

 planes of division in successive generations is a phenomenon inde- 

 pendent of any system of nomenclature of " spirals." It is a 

 factor, however, which in my opinion should be given weight in 

 any system of nomenclature. If we give to the spiral of the 

 second quartet the designation hitherto universally applied to it, 

 we must call it a right spiral, and thus ignore the factor of alterna- 

 tion, for that of the first quartet and same generation is called a 

 left spiral. If, on the other hand, we recognize this similarity in 

 the direction of the obliquity of the spindles of both quartets 

 of this generation, we should give their spirals the same name. 

 This is accomplished by referring the nomenclature of the spiral 

 to the basis suggested in the preceding pages, i. e. by regarding 

 the lower of the two daughter cells as fixed, the upper one as 

 movable or diverging. In the case of the two dividing quartets 

 under discussion the upper asters of both quartets are the left 

 asters, and consequently of the daughter cells the upper is the 

 left ; i. e. the direction of the divergence or rotation is toward the 

 left, or opposite the direction in which the hands of a watch 

 move, and the spirals in both quartets should be called left 

 spirals. 



The Sixth Generation is produced by the division of the 

 four quartets of the fifth generation, a 51 - d 6 \ a 52 - d 52 , a 6 - 8 - d 6 - 3 , 

 and a 5A - d 5A , which results in the formation of the eight quartets 

 a 61 - d*- 1 , a 62 - d 6 -% a 63 - d 6S , a 6A - d« A , a 6 - 5 -d™, a 6 - 6 - d G6 , a 67 - d M , and 

 a 6 - 8 - d* 8 . 



These divisions are not synchronous, however, neither are they 

 all accomplished before the divisions resulting in the succeeding 

 (seventh) generation begin ; for from this time on in the history 

 of the egg successive generations of cells overlap one another, so 

 that we find in the egg at the same time cells belonging to two 

 or more generations. In every case the cell is designated as 



