38 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



tlio 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 egg and embryo, which is one of the 

 must 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 {eg. the letter designating each of the cleavage cells of the ninth generation, 

 figs. 140-143, wotdd need to be followed by no less than six exponents). Owing to 

 these reasons I 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). In this sys- 

 tem, as is w^ell 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 

 one; 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 one" (Castle. 

 L896, [>. 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. viz. A and 1\ 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 cells 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-eell stasre 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 



