186 PROCEEDINGS OP THE AMERICAN ACADEMY. 



The advantages of this system are as follows. (1) It does not 

 involve the confusion of designating two or more different cells 

 by the same characters, as is the case when a daughter cell is 

 given the same designation that the mother cell had ; e. g. when 

 the macromere A divides, and the derivatives are named A 

 and a 1 and the same designation is employed for the basal cell 

 in the succeeding divisions. (2) Never more than two ex- 

 ponents need be employed, and practically the limit of three 

 figures is not exceeded. We thus avoid the cumbersome and 

 confusing exponents which characterize the late periods of 

 cleavage in other systems ; e. g. a" 12A of Heymons ('93, p. 259, 

 Taf. XV. Fig. 20) becomes a 9 - 25 . (3) The designation affords 

 some clue to the relative position of the cell and the quartet to 

 which it belongs. Thus the quartet with the second exponent 1 

 is always at the vegetative pole of the egg. The apical quartet 

 is always designated by one of the even numbers 2, 4, 8, 16, 32, 

 etc. The exponents of any cell give a hint as to the designation of 

 the adjoining cells. Thus, ft 6 - 8 of Figure 3 (Plate I.) lies in contact 

 with a 5 - 3 and b b ' s and b 6,1 . (4) The derivation of a cell is implied 

 in its designation in every case. Thus, b 6 ' 8 is derived from the 

 cell b of the fifth generation and fourth quartet, i. e. from b 5A , b 5A 

 from & 4-2 , and 6 4,2 from b 3 . The mother cell of any given cell is 

 always designated (1) by the same letter, (2) by the first expo- 

 nent of the daughter cell less one, (3) by one half of the second 

 exponent when that exponent is even, or by one half the sum of 

 the second exponent and one when it is odd. Thus, b 6J and b 6 - s are 

 derived from b 5A . In like manner, the designation of the mother 

 cell determines that of the daughter cells ; for example, when 

 b*- 2 divides, the daughter cells are always designated as b 5 - 3 and 

 b 5A , ivhatever their 'position and relation to other cells or quartets. 

 In typical cleavage they will be members of the third and fourth 

 quartets from the vegetative pole. This typical condition is, how- 

 ever, sometimes modified by delayed cleavage, or by the distribu- 

 tion of the yolk. 



This system of nomenclature may also be extended to the 

 cleavage of the bilateral period, and to the radial type of cleav- 

 age. In the case of cleavage in a plane parallel to the equator, 

 the upper and lower daughter cells may be designated as in the 

 spiral period and type of cleavage. In meridional cleavages the 

 right derivative may be designated by the even exponent and 

 the left by the odd in the case of even generations, and the reverse 



