182 PROCEEDINGS OF THE AMERICAN ACADEMY. 



constant basis of reference. Not only have these authors named 

 homologous spirals differently, as Lang ('84, p. 325) and Heymons 

 ('93, p. 256), but no one of those named except possibly Lang has 

 used the same method of naming all the spirals discussed. In 

 some cases the lower cell of a pair of daughter cells is regarded 

 as the fixed one, in other cases the upper cell is so regarded ; or, to 

 express it differently, in some cases peripheral cells are regarded 

 as fixed, in other cases axial ones. In general it seems to have 

 been the custom to consider the larger of the daughter cells as 

 fixed, and the budding smaller cell as the movable one. Two 

 reasons may be cited for the employment of the relative size of 

 the daughter cells as a basis for the nomenclature of the spiral. 

 (1) The larger cell occupies more nearly the position of the 

 mother cell, and it is therefore natural to regard the smaller 

 cell as the movable one. (2) In the first spiral the larger cells 

 (macromeres) are basal, and the micromeres upon them are there- 

 fore regarded as the movable cells, and this basis adopted in the 

 first spiral is suggested for other spirals. Though this refer- 

 ence of the spiral to the relative size of the cell may furnish a 

 logical basis for nomenclature of spirals where cleavage is un- 

 equal, it cannot furnish one for those spirals, or eggs, in which 

 cleavage is equal. Nor has this basis when once adopted been 

 consistently followed in every case, — as, for example, in Wilson's 

 paper on Nereis ('92). On page 391 he says: " A careful study 

 of the embryo through these changes shows that all of the cell 

 divisions conform to the spiral type. . . . It is also easily seen 

 in the divisions of the secondary micromeres (a 2 , b 2 , c 2 , X). Each 

 of them divides somewhat obliquely (cf. Figs. 25, 26, 33) so that 

 one of the cells lies somewhat lower than the other, and in most 

 cases the lower cell is obviously smaller than the upper. The 

 difference in size is very great in the case of X and x', but is much 

 less in the case of the others (a 21 , a 22 , Fig. 33). (In the speci- 

 men shown in Figs. 25, 26, on the other hand, there is no appre- 

 ciable difference in size, but I have never seen a case in which the 

 upper cell is the smaller.) If this group of cells be followed 

 around the embryo from right to left (against the hands of a 

 watch), the upper (larger) cell always comes first ; i. e. the first 

 division of the second group of micromeres takes place in a 

 left-handed spiral, like the second division of the first set of 

 micromeres." 



There is no escape from the conclusion that in this case the 



