38 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 
the latter there is no distinction of macromeres and micromeres, because there are 
no “quartets” which arise from stem cells, because this system is not well adapted 
to show the perfect bilateral symmetry of the ege and embryo, which is one of the 
most characteristic features of ascidian development, and finally because of the great 
number of figures which must be used as exponents in the representation of later 
stages (e g. the letter designating each of the cleavage cells of the ninth generation, 
figs. 140-143, would need to be followed by no less than six exponents). Owing to 
these reasons І early saw the difficulty of attempting to apply this system to the 
cell-lineage of the ascidian egg. The most complete system of nomenclature which 
has heretofore been used in the study of the cell-lineage of ascidians is that of 
Castle, which is a modification of a system devised by Kofoid (1894). Іп this sys- 
tem, as is well known, the four quadrants of the egg are designated by the letters 
A, B, C, D; after the third cleavage the cells nearer the vegetal pole are designated 
by capitals, those near the animal pole by lower case letters. The first expon- 
ent following a letter indicates the generation to which a cell belongs, the second 
exponent the position of the cell relative to the vegetal pole. With this system it 
is always difficult to determine at a glance the lineage of any cell since “ to ascer- . 
tain the mother cell of any particular cell, its first exponent must be diminished by 
опе; and its second exponent, if an even number, must be divided by two, but if 
an odd number it must be first increased by one and then divided by two. In 
order to determine the daughter cell of a particular cell, simply reverse this process ; 
that is, increase the first exponent by one and double the second exponent. To 
determine the other daughter cell diminish this second exponent by опе” (Castle, 
1896, p. 227). While it is thus difficult to determine at a glance the lineage 
of any cell, the number of exponents required is relatively small, and this fact, 
more than any other, has led me to adopt Castle's system, with the following modi- 
fications :— The right and left halves of the embryo are designated by the same 
letters, the names of cells on the right being underscored as compared with those 
on the left. This method of designating the cells of the right and left sides is 
essentially similar to that employed by Chabry (1887). In this way but two 
letters are needed for the whole cell-lineage, one for the anterior and another for 
the posterior quadrants. For these I desired to use the letters employed by 
Van Beneden and Julin and by Chabry, zzz. A and P, but owing to the diffi- 
culty of distinguishing between lower case and capital P, I finally chose the letter 
B instead of P. The right anterior quadrant is A, the left A; the right pos- 
terior quadrant is B, the left B. After the third cleavage all өсі lying on the 
polar body side of that cleavage plane are designated by lower case letters, while 
those on the opposite side of that plane continue to be designated by capitals. This 
modification not only emphasizes the bilateral character of the ascidian egg, but it 
also simplifies the nomenclature. Furthermore, it facilitates reference to Castle’s 
work, for when in his orientation of the 48-cell stage the egg is inverted as com- 
pared with earlier stages the right side is substituted for the left, and the letters A 
and D, which in the earlier stages designate the actual left side, are used after the 
