16 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Zelinka’s orientation were adopted, the point where the polar cell is 
formed might be first ventral, then posterior, then dorsal, and later an- 
terior, and this would actually be the case in Asplanchna. In a special 
study of cleavage, with particular attention to direction, it is necessary 
that the orientation should give some constant basis for reference. It is 
therefore impossible for me to use Zelinka’s orientation in my work. 
The animal pole of the egg does retain, however, a constant relation to 
the position of the blastomeres and to the axes of cleavage, so that I 
have adopted this relation as a basis for orientation. 
4, CLEAVAGE. 
Nomenclature. 
For accurate comparative study of the direction and sequence of cleav- 
age in the different regions of the egg, such a system of nomenclature is 
needed as will indicate directly the relationships, and especially the com- 
parative age (measured in cell generations) of the blastomeres. The only 
system of nomenclature hitherto proposed which fulfils these demands is, 
I believe, that of Kofoid (94). I shall therefore use his system in the 
following account. 
The four blastomeres of the four-cell stage, and the cells derived 
from them, are designated respectively by the letters A, B, C, and D, 
beginning with the left anterior blastomere and passing around the egg 
to the right, i.e. in the same direction as the hands of a watch, — as- 
suming the egg to be viewed from the animal pole. The letters thus 
represent the same blastomeres as iu Wilson's work (92) on Nereis, 
Heymons’s (93) on Umbrella, Lillie's (95) on Unio, and Kofoid’s (95) 
on Limax. 
After the first equatorial cleavage, at which the four original blasto- 
meres are divided into smaller cells, the capital letters A, B, C, and D 
will be reserved to indicate respectively all the cells derived from the 
corresponding cell of the first four blastomeres ; and such a collection 
will be called a quadrant of the egg.“ The separate cells will be desig- 
nated by the lower-case letters a, b, c, and d, according to tho quadrant 
to which they belong. Each letter will be followed by two exponents. 
The first exponent indicates the generation to which the cell belongs, 
the unsegmented egg being considered the first generation. "Thus, in 
1 Each quadrant from the four-cell stage onward receives a specific color in the 
plates, so that the quadrants are instantly distinguishable by their colors. 
