184 PROCEEDINGS OF THE AMERICAN ACADEMY. 



hand spiral. This is the only case in which the similarity 

 of the position of the spindles is recognized as a factor in the 

 nomenclature of spirals. By this change in the basis of nomen- 

 clature he has recognized the interesting fact that there exists in 

 the three successive divisions of the primary micromeres (a 1 , b 1 , 

 c 1 , d 1 ) an alternation in the direction of the spirals. 



This alternation, as described and named by him, is inde- 

 pendent of that which exists in the first three divisions of the 

 macromeres. It belongs to an entirely separate system, and its 

 relation to successive generations of cells is neither suggested 

 nor discussed. 



Not only does the system of nomenclature based on the size of 

 the cells fail to furnish a logical basis for cases of equal cleavage, 

 but it also fails to furnish such a basis for the comparison of 

 cleavage in different forms, for the cells in which the yolk is 

 lodged in the progress of cleavage are by no means homologous 

 cells in different species (cf. Nereis, Neritina, and Umbrella). 

 It therefore seems to me very much more logical to base the 

 nomenclature of spirals upon constant spatial relations than upon 

 the relative sizes of the daughter cells, which are inconstant, or 

 upon the apparent greater shifting of one of the daughter cells 

 referred to no constant plane. 



The student of cell lineage finds his task much complicated by 

 the various systems of nomenclature already employed, and it 

 seems a pity to introduce still another to add to the confusion. 

 But as all systems heretofore employed fail to recognize the 

 fundamental importance of sucessive generations of cells, and as 

 these are the basis of my treatment of the subject, it has been 

 impossible to adopt any of the existing systems. At the same 

 time, it is believed that the system proposed, resting as it does 

 upon generations of cells, is adapted to all forms of spiral and 

 radial cleavage, and will furnish a satisfactory and convenient 

 means of comparison in these cases. Its applicability to spiral 

 cleavage will now be discussed more fully. 



Each individual cell of an egg in spiral cleavage can be traced 

 back to one of the four blastomeres a, b, c, d, i. e. it belongs to a 

 definite quadrant. It also belongs to a definite quartet or " story " 

 of the egg; it is likewise removed from the ovum by a definite 

 number of cell divisions, i. e. belongs to a definite generation. 



Any system of nomenclature involving these three factors will 

 both localize the cell and outline its ancestry. Each cell of the 



