42 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



with a view of meeting this objection and suggesting a suitable basis of 

 comparison that I proposed, in a paper ('94) to which I must refer the 

 reader for a detailed description, an entirely new principle of nomencla- 

 ture of both cells and spirals. This is a strictly genealogical system, 

 giving to each cell in the line of descent a separate designation, one de- 

 termined moreover by the constant spatial relations common to all eggs, 

 and not by the inequality of the cleavage characteristic of individual 

 species. 



The system presupposes the division of the egg into four quadrants 

 designated a, b, c, d, placed in the order in which the hands of a clock 

 move. These quadrants are occupied by the four blastomeres, the 

 quartet, of the third generation. When this quartet, or any other quar- 

 tet of the later stages, divides, forming two quartets, each cell is desig- 

 nated as follows : (1) by a letter indicating the quadrant, as, e. g., a ; 

 (2) by a first exponent indicating the generation, a 3 , a*, etc. ; (3) by a 

 second exponent indicating the position of the quartet with reference to all 

 other quartets of the same generation, potential or actual, the quartets 

 being numbered from the vegetative toward the animal pole, as a 41 , a 4-2 , 

 etc. Thus the cell a 3 divides, forming a 4 " 1 and a* 2 ; in the second ex- 

 ponent the odd one being always given to the cells of the quartet which is 

 nearer the vegetative pole, and the even to those of the quartet nearer the 

 animal pole. I have previously described ('94) the simple and constant 

 manner in which the designation of the daughter cells can in every case 

 be dei'ived from that of the mother cell. 



It may be well to call attention here to the significance of this system 

 of nomenclature. It designates cells as they might be named in the 

 simplest possible mathematical and mechanical conditions of a cleaving 

 egg, i. e. equal, regular cleavage pervading all the cells of a generation 

 at the same time. In such a case we should have all the quartets of a 

 generation actually present and numbered in the regular succession of 

 their position from the vegetative to the animal pole. The possibility 

 of referring all forms of spiral cleavage to such a simple type is obvious* 

 and the advantage, if not indeed the necessity, of such a reference as a 

 basis of comparison is equally apparent. The fact that in the applica- 

 tion of this system the exponents have little or no significance, or are 

 even misleading, as to the actual number of quartets present between a 

 given quartet and the vegetative pole, is thus no obstacle, when once 

 the real significance of the system is understood. In fact, it is rather 

 an advantage that the regions of most rapid growth in the embryo are 

 thus prominently designated. There are doubtless objections that 



