76 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
The system devised by Kofoid ('94) — which Castle applied to the 
bilateral cleavage of tunicates, where the conditions of cleavage resemble 
those of Lepas — has with some necessary modifications been followed. 
The cells of the four-cell stage are designated a, 6,c and d in the usual 
order, a being the left anterior cell. An exponent indicates the number 
of the generation, starting with the ovum as the first, e. g. a*, 6%, etc. 
A second exponent is used to distinguish a cell from other cells of the 
same generation and derivation, e. g. a*", a*?, a**, etc. In assigning 
the second exponent I have followed in part suggestions made by Kofoid 
(94) and put into practice by Castle (96). In cases of equatorial 
division the odd numbers have been applied to the cells nearer the vege- 
tative pole, and the even to those nearer the animal pole. Thus of the 
cells in the four-cell stage a® divides, forming a*' which is nearer the 
vegetative, and a‘? which is nearer the animal pole, while its sister cell, 
6°, forms 64) and b*:? (see Plate 4, Figs. 34-38). In later stages, where 
cells do not divide equatorially, but parallel to the sagittal plane, the odd 
exponent has been applied to the cell lying nearer that plane. In cases 
where a cell lies in the sagittal plane and undergoes division in the same 
plane, the daughter cell on the 7zght side of that plane is designated by 
the odd exponent. Whenever cells divide transversely to the chief axis 
of the embryo, the anterior cell is designated by the odd exponent. 
In determining the designation of cells, the rules given by Kofoid are 
here applied to Lepas. The designation of any derivative of cells a, b, 
ce, d being given, the designation of mother cell or daughter cells can be 
quickly determined. The first exponent indicating the generation of the 
mother cell will, of course, be one less than that of the daughter cell. 
The second exponent of the mother cell will be one-half of that of the 
daughter cell, if that be an even number, and one-half the sum of the 
second exponent plus one, if that be an odd number. Thus a*? and a*? 
are daughter cells of a*. Likewise, to determine the first exponent of 
the daughter cells, add one to the first exponent of the mother cell; to 
determine the second exponent, multiply the second exponent of the 
mother cell by two and the product is the designation to be applied to 
the cell bearing the even number as exponent, while that product less one 
designates the sister cell. Thus a5 ° dividing forms a®? and a®"}. 
A summary of the important points in the cell-lineage of Lepas is given 
in a table in connection with the general summary. 
