JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 17 



the eight-cell stage (fourth generation) we shall have cells a*, J*, c*, 

 and d\ Since, however, in this and in later generations, there are 

 more than one cell of a given quadrant in a given generation, this first 

 exponent must be followed by a second, serving to distinguish each cell 

 from every other of the same quadrant. In " spiral" cleavage, this sec- 

 ond exponent indicates the "quartet," or layer of cells, in the embryo 

 to which the blastomere belongs, the ventral cell being number 1, the 

 next dorsal number 2, and so to the most dorsal quartet. In equatorial 

 cleavages the same relation may be preserved in other types of cleavage 

 than the spiral. Thus, in the eight-cell stage (fourth generation), the 

 ventral blastomeres are a*'^, b*'^, c*-^, and d*-^, while the corresponding 

 dorsal cells are a*-% b^'^, c*-^, and rf^-^. But in meridional cleavages, 

 where there is no trace of the so called spiral, this criterion fails, and 

 the second exponent can be used only for distinguishing the cells, not 

 for indicating their relative positions. What is required is a rational 

 system of applying the exponent such that no two cells of the same 

 quadrant in the same generation shall have the same exponent. Follow- 

 ing the suggestion of Kofoid ('9-i), I have in meridional cleavages desig- 

 nated tlie right derivative with the even exponent in even generations, 

 and with the odd exponent in odd genei'ations, — the left derivative of 

 course receiving the reverse designation. This method of application 

 was designed to preserve any possible homologies of the products of 

 meridional with those of spiral cleavage, since in normal spiral cleavage 

 the right derivative lies above the left in even generations, and so re- 

 ceives the even exponent, while in odd generations the reverse is true. 

 The results, however, have not shown any striking homologies with 

 spiral cleavage, but the method of application has been retained, since 

 no other seems to have any advantage over it. 



In meridional cleavages, the terms right and left will be used as de- 

 fined by Kofoid ('94, p. 180) : "A miniature observer is imagined as 

 placed in the principal (vertical) axis of the egg, with his head at the 

 animal pole, facing the part or parts under consideration, and the terms 

 right and left, upper and lower, are used as determined by this observer." 



A full account of this system of nomenclature is given by Kofoid 

 ('94). In order to make clear the relation of the succeeding blasto- 

 meres and their designations by this system, I give here a scheme of the 

 nomenclature thi-ough the sixth generation, modified from that given by 

 Kofoid. Only the products of quadrant A are carried out beyond 

 the third generation, since the method is the same for the other quad- 

 rants. Here a®-^ represents the most ventral cell derived from the 



VOL. XXX. — NO. 1. 2 



