castle: embryology of giona 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 A, the right anterior B, the right posterior C, and 

 the left posterior B. In dorsal views, A and B 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 indicates the number of a cell in a generation, 

 the cells of each quadrant being numbered independently from the animal 

 towai'd 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 Jirst exponent 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 

 of a^'^ are a^-^ and a®'^. 



2. Cleavage. 



A. Early Stages of Cleavage. 



(rt) To 2Jf-cell Stage. 



Figures 19-26 (Plate IV.) show eight views of a living egg, drawn by 

 means of an Abbe camera lucida at successive stages, the egg remaining 

 undisturbed in position under the microscope throughout the period of 

 observation. The left side of the egg is, as I shall show, towards the 



^ In 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- 

 tative 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. 



