No. I.] THE EMBRYOLOGY OF CREPIDULA. 35 



nated by coefficients rather than by exponents ; e.g., the first 

 quartette of micromeres and all their derivatives are desig- 

 nated by the coefficient i (la, id, Ia'•^ ic^% etc.), the second 

 quartette and its progeny by the coefficient 2 (2a, 2d, 2c3S 

 etc.), the third quartette by the coefficient 3 (3a, 3d, etc.), 

 and the fourth quartette by 4 (4a, 4d, etc.). I emphasize this 

 difference between the quartettes of micromeres because in 

 general their histories are very different, and also because it 

 is only by following the different quartettes that I have been 

 able to trace the cell lineage in the more advanced stages. 



Another and an all-sufficient reason for emphasizing in the 

 nomenclature the different groups or quartettes separated from 

 the macromeres, is the fact that, so far as known, the same 

 number of quartettes with essentially the same destiny is sepa- 

 rated in all annelids and mollusks with holoblastic segmenta- 

 tion. This is certainly a feature of great morphological 

 importance, and deserves special recognition in the nomencla- 

 ture. This system of nomenclature will be better understood 

 by reference to the following cytogenetic table. 



The animal and vegetal poles are considered the fixed points 

 in the &gg. In the ectoblast the stem or parent cell is in all 

 cases the upper one. The stem cell in the entoblast and meso- 

 blast is in every case the lower one. If, in any case, the cleav- 

 age is perfectly meridional (an exceedingly rare thing), the 

 right moiety is considered the stem cell. The terms iight and 

 left are employed in the usual sense, i.e., right is clockwise, 

 left is anti-clockwise. A cleavage is oblique to the right, or, 

 following Lillie ('95), dexiotropic, when the upper moiety lies 

 to the right of the lower ; it is oblique to the left, or laeotropic, 

 when the upper moiety lies to the left of the lower. The direc- 

 tion of a cleavage refers to the direction of the nuclear spindle, 

 not to the plane of the division wall. 



of the egg. In numbering the different quartettes, however, I have departed 

 somewhat from Kofoid's system. The four macromeres are the basal quartette ; 

 the first group of ectomeres separated from these are the first quartette, the 

 second group the second quartette, etc. 



