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 (794) 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., « 
(2) by a first exponent indicating the generation, aè, 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*!, a*?, 
etc. Thus the cell a? divides, forming a*' and a*?; 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 derived 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 
