KOFOID. — LAWS OF CLEAVAGE. 183 



cell of reference is the larger one, as it was in the case of the 

 " third and fourth cleavages " (cf. Wilson, pp. 387 and 388). That 

 it is the " larger " rather than the " upper " cell, will be seen when 

 in Wilson's Fig. 21 (reproduced in outline in my Plate I. Fig. 5) 

 we apply to the spirals foreshadowed in the spindles of the cells 

 d l , X (— d' 2 ), and D, the method employed by him in naming this 

 spiral (a 2 , b 2 , c 2 , X). Following around the embryo from right to 

 left the "upper " cell (indicated by the upper end of the spindle) 

 comes first in all three cases, but the spirals are not all given the 

 same name. The "larger" cell comes first in d 1 and X, and the 

 spirals are called left-handed spirals. The smaller cell comes first 

 in D, and the spiral is called a right-handed spiral (p. 391). — Let 

 me call attention, in passing, to the fact that, in the system of 

 nomenclature I have proposed, the three spirals above referred to 

 would be given the same name. They would all be called right 

 spirals, and in this similarity of name would be recognized the 

 similarity of the position of spindles, and the fact that in passing 

 from right to left the upper cell always comes first. The basis on 

 which my system rests is not the varying size of the cells, but the 

 more fundamental factor of position. — Up to this point in cleav- 

 age Wilson has consistently used his system of nomenclature, 

 but upon the next page (p. 392), in discussing the third division 

 of the primary micromeres a 1 , b 1 , c 1 , d 1 , resulting in the formation 

 of the cells a 1 , b 1 , c 1 , d\ and the rosette cells a 13 , b lz , c 13 , d 13 , 

 he abandons the larger cell as the basis of reference, as will be 

 seen in the following quotation : " The four primary micromeres 

 (a 1 , b 1 , c 1 , d 1 ) bud forth four small cells at their inner angles (at 

 the upper pole) which arrange themselves in a very regular apical 

 rosette, the cells of which alternate with the central micromeres 

 (Figs. 27, 28, etc.). The position of the spindles is the same as 

 in the first division of a 1 , b 1 , c 1 , d 1 ; i. e. the division follows a 

 right-handed spiral, but the character of the division is very 

 different since the smaller cells are formed at the central instead 

 of the peripheral angles of the cells (i. e. towards instead of 

 away from the vertical axis of the embryo)." In this case the 

 fact that " the position of the spindles is the same " is cited as 

 a basis on which the spiral is named. In my Fig. 6, Plate I., is 

 reproduced in outline Fig. 25, Plate XV., of Wilson's paper. If we 

 apply to the cells c 1 and c 13 the test mentioned by Wilson on 

 page 391, and name the spiral according to the size of the cell 

 that " comes first," we must call it, not a right-hand, but a left- 



