JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. ale 
the eight-cell stage (fourth generation) we shall have cells af, bt, c$, 
and d*, Since, however, in this and in later generations, there are 
more than one cell of a given quadrant in a given generation, this first 
exponent must be followed by a second, serving to distinguish each cell 
from every other of the same quadrant. In “ spiral” cleavage, this sec- 
ond exponent indicates the “quartet,” or layer of cells, in the embryo 
to which the blastomere belongs, the ventral cell being number 1, the 
next dorsal number 2, and so to the most dorsal quartet. In equatorial 
cleavages the same relation may be preserved in other types of cleavage 
than the spiral. Thus, in the eight-cell stage (fourth generation), the 
ventral blastomeres are d, 0^, c., and d, while the corresponding 
dorsal cells are a, 0*2, , and d. But in meridional cleavages, 
where there is no trace of the so called spiral, this criterion fails, and 
the second exponent can be used only for distinguishing the cells, not 
for indicating their relative positions. What is required is a rational 
System of applying the exponent such that no two cells of the same 
quadrant in the same generation shall have the same exponent. Follow- 
ing the suggestion of Kofoid (94), I have in meridional cleavages desig- 
nated the right derivative with the even exponent in even generations, 
and with the odd exponent in odd generations, — the left derivative of 
course receiving the reverse designation. This method of application 
was designed to preserve any possible homologies of the products of 
meridional with those of spiral cleavage, since in normal spiral cleavage 
the right derivative lies above the left in even generations, and so re- 
ceives the even exponent, while in odd generations the reverse is true. 
The results, however, have not shown any striking homologies with 
Spiral eleavage, but the method of application has been retained, since 
no other seems to have any advantage over it. 
In meridional cleavages, the terms right and left will be used as de- 
fined by Kofoid (94, p. 180): “A miniature observer is imagined as 
placed in the principal (vertical) axis of the egg, with his head at the 
animal pole, facing the part or parts under consideration, and the terms 
right and left, upper and lower, are used as determined by this observer." 
A full account of this system of nomenclature is given by Kofoid 
(94). In order to make clear the relation of the succeeding blasto- 
meres and their designations by this system, I give here a scheme of the 
nomenclature through the sixth generation, modified from that given by 
Kofoid. Only the products of quadrant .4 are carried out beyond 
the third generation, since the method is the same for the other quad- 
rants. Here a9! represents the most ventral cell derived from the 
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