KOFOID. — LAWS OF CLEAVAGE. 181 



any given set may or may not coincide with that of the other 

 sets of the same generation ; but whatever the modifications, each 

 cell in its origin bears a close relation to three other cells, and 

 these sets of four related cells of co-ordinate origin will be called 

 quartets. During spiral cleavage the egg is made up of a num- 

 ber of superposed quartets, and it may be compared to a house 

 of as many stories, each story representing a single quartet of 

 four cells. 



The regions of the cleaving egg occupied by the four blasto- 

 meres of the four-cell stage and their derivatives during the 

 spiral period will be called quadrants, and the four primitive 

 blastomeres and their respective derivatives will be designated 

 by the letters a, h, c, d, taken in the order in which the hands 

 of a clock move. In Nereis and Umbrella, these designate the 

 left anterior, right anterior, right posterior, and left posterior 

 quadrants respectively. 



The term spiral will be used to indicate the divergence imme- 

 diately after cleavage of the centre of the nucleus of the upper 

 one of two daughter cells from the vertical plane passing through 

 the corresponding portion of the lower cell and the vertical axis 

 of the egg. The spiral will be a right spiral when the divergence 

 is toward the right (as defined above), and a left one when the 

 divergence is toward the left. Or, using Blochmann's ('81) com- 

 parison to the hands of a clock, when the egg is observed from the 

 animal pole the spiral is a " right " one if the divergence of the 

 upper cell is in the direction of the motion of the hands of the 

 clock, a "left" spiral if the divergence is opposite the motion 

 of the hands of the clock. By this method of nomenclature all 

 those spirals whose spindles stand in similar positions with 

 reference to the vertical axis are given the same name. 



It should be noted in this connection, that this divergence, 

 or apparent shifting of cells, in Limax at least, is the result of 

 the obliquity of the plane of division, and is predetermined by 

 the position of the spindle. This fact, whatever may be the cause 

 of the particular position of the spindle, is the immediate basis 

 of the phenomenon termed the "spiral." The position of the 

 spindle primarily determines the position of the daughter cells, 

 though mechanical environment may secondarily modify that 

 position. 



The nomenclature of spirals as followed by Blochmann ('81), 

 Lang ('84), Wilson ('92), Heymons ('93), and others, presents no 



