CASTLE. — CELL LINEAGE OF THE ASCIDIAN EGG. 203 



In any extended work on cell lineage it is desirable to have some 

 system of naming the individual cells which will indicate readily the 

 exact history of each, — from vvliat part of the matured ovum it has 

 been derived, by how many divisions it is removed from the ovum, 

 and from what other cells these divisions have separated it. 



Wliat system one adopts is a matter of choice, but not of indiffer- 

 ence. Chabry has employed a fnirly good one in his work on Ascidi- 

 ella. It is too cumbersome, however, for advanced stages, and limited 

 in its applicability. Van Beneden et Juliu have simply employed 

 numerals, which give no information whatever as to the derivation of 

 cells. Seeliger has followed no system at all beyond the 16-cell 

 stage, except that of arrows joining cells of the same parentage, 

 which serves to mark the lineage for only one generation. 



So far as I know, only one system capable of general application 

 to different types of cleavage has been proposed, that introduced by 

 Kofoid ('94) in his recent work on Limax. As this seems to me 

 to embody several distinct advantages over other systems, I shall 

 follow its general features in this paper. 



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 ectodermal side, the left anterior quadrant is A, the right anterior 

 B, the right posterior C, and the left posterior D. In dorsal views, 

 right and left are of course reversed. 



As the third cleavage plane is equatorial and separates a ventral 

 from a dorsal hemisphere, I shall designate the cells of the former by 

 capitals and those of the latter by small letters. 



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 unsegmented ovum. 



The ovum is generation one, the 2-cell stage two, the 4-cell stage 

 three, etc. 



4. The second exponent indicates the niiviber of a cell in a genera- 

 tion, the cells of each quadrant being numbered independently of the 

 other quadrants from the centre of the ventral (ectodermal) toward the 

 centre of the dorsal side.* If in any case two cells of common de- 



* In tliis I do not follow Kofoid, who numbers from the ventral (in his case 

 endodermal) toward the dorsal (in liis case ectodermal) pole. His system of 

 nomenclature I have treated throughout as being, for my purposes, not an index 

 to homologies between blastomeres, but a convenient method of notation. 



