CASTLE: EMBRYOLOGY OF CIONA INTESTINALIS. 227 
follow with some modifications the system introduced by Kofoid (94) in 
his work on Limax. 
1. Each cell will be designated by a letter with two exponents. 
2. The letter indicates the quadrant of the egg from which the cell in 
question has been derived, or in other words that cell of the 4-cell stage 
from which it is descended. Viewing the egg from the ventral or animal 
pole (the one opposite that at which the polar cells are formed), the left 
anterior quadrant is 4, the right anterior Ø, the right posterior C, and 
the left posterior D. In dorsal views, 4 and D are of course the right 
quadrants, and B and C the left. 
3. The first exponent indicates the generation to which a cell belongs ; 
that is, the number of cell divisions by which it is removed from the 
ovum. The ovum is generation one, the 2-cell stage two, the 4-cell stage 
three, etc. (See the Table of Cell Lineage on page 275.) 
4. The second exponent indieates the number of a cell in a generation, 
the cells of each quadrant being numbered independently from the animal 
toward the vegetative pole.! If in any case two cells of common descent 
lie in an equatorial position, that one which is nearer the sagittal plane 
is given the lower numeral. 
To ascertain the designation of the mother cell of any particular cell, 
its first oxponent must be diminished by one; and its second exponent, 
if an even number, must be divided by two, but if an odd number it 
must first be 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 one. For example, the daughter cells 
ot ad are a6 and al”, 
2. Cleavage. 
A. EARLY SrAGES or CLEAVAGE. 
(a) To 24-cell Stage. 
Figures 19-26 (Plate IV.) show eight views of a living egg, drawn by 
means of an Abbé camera lucida at successive stages, the egg remaining 
undisturbed in position under the mieroscope throughout the period of 
observation. The left side of the egg is, as I shall show, towards the 
! Tn gastrulation, the cells about the vegetative pole are depressed to a lower level 
than the margin of the blastopore. In naming cells it is considered that the vege- 
lative pole is also depressed at that period, and lies constantly on the dorsal surface 
at the common point of meeting of the cells derived from the four quadrants. 
