CASTLE. — CELL LINEAGE OP THE ASCIDIAN EGG. 205 



the terminology of Van Beneden et Julin, the cell liueage of the 

 44-cell stage as interpreted by tliem ; and in the succeeding column 

 this interpretation is repeated in terms of the nomenclature adopted 

 hv me. Finally, in the last column is given my own interpretation 

 of the same stage. 



The cell lineage for the left half of the egg would be expressed by 

 substituting, in the table, B, b, for A, a, and C, c, for D, d, where 

 my terminology is used ; and where the terminology of Van Beneden 

 et Julin is used, by adding a (') to each of the numerals. 



To ascertain the designation of the mother cell of any particular 

 cell, its first 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 cells of a particular cell, simply 

 reverse this process ; that is, increase the first exponent by one and 

 double the second exponent. The result will be the name of the 

 daughter cell having an even second exponent. To determine the 

 other daughter cell, diminish this second exponent by one. Thus the 

 daughter cells of a^* are a®'^ and a'^-^ 



This system I have applied to the 32-cell stage of Van Beneden 

 et Julin in Figures 7 and 8, reproduced from their Figures 9 and 9a, 

 the orientation being corrected to agree with my interpretation. 



Similar views of the same stage of Ciona are given in Figures 

 3 and 4. 



It will be seen that this stage of Ciona corresponds cell for cell 

 with the one of Claveliua figured. 



In both, the posterior end is marked on one side (Figs. 3 and 7) by 

 the superficially, if not actually, largest pair of cells in the Qgg at this 

 stage, while underneath them, best seen from the other face of the 

 egg, is the smallest pair of cells (Figs. 4 and 8). The only thing in 

 the egg of Ciona figured which is not perfectly typical is the position 

 of the cells a*'-^ and b*'-^, which in other specimens examined lie 

 anterior quite as often as lateral to their sister cells, a^^ and b*'® 

 respectively. 



A nearly median optical section parallel to the sagittal plane of the 

 egg exhibited in Figures 7 and 8 is reproduced in Figure 13, and an 

 actual transverse section of the same stage of Ciona is represented in 

 Figure 15. It will be seen that in both cases the cells of one face, 

 which are apparently very large when viewed from the surface, are 

 really thin and superficial ; while the apparently small cell? of the 

 other face are really high, columnar, and of considerable volume. 



