KOFOID: development of UMAX. 57 



nomenclature of regarding the larger of two (laughter cells as the mother, 

 tlie smaller as the daughter. In reality both are daughter cells, and 

 the mother cell has passed out of existence with their origin, a fact 

 which a logical system of nomenclature of cell lineage must always 

 recognize. 



Sixth Generation. Thirty-two Cells. 



Following the formation of the sixteen cells of the previous generation 

 there comes the usual rounded condition in which each blastomere asserts 

 its individuality and diverges from its nearest of kin. This in turn is 

 followed by the flattened condition, in which the spherical contour of 

 the egg as a whole is restored. It is during this period that the spindles 

 whidi begin the formation of the sixth generation first appear. As in 

 the previous generation there was a lack of synchronism in the cleavage 

 of the two quartets 4.1 and 4.2, as shown in the nuclear conditions of 

 Figures 20 and 21 (Plate III.), so here there is a similar separation 

 of the divisions of this generation mto two mitotic periods, the first 

 appearing in the largest cells of the embryo, viz. the two quartets at 

 the vegetative pole, and resulting in the twenty-four-cell stage ; the 

 second involving the animal hemisphere, and resulting in the thirty-two- 

 cell stage, thus realizing in this stage Wainieck's ('50) conclusion that 

 cleavage progresses according to the age of the cleavage spheres. The 

 first phase is separated from the second by a period in which all the 

 nuclei are in a resting condition. This period lasts some hours, and 

 hence it is that cleavage stages killed at random contain a large pro- 

 portion of eggs in the twenty-four-cell stage. 



Division of Quartet 5.1, forming 6.1 and 6.2. 



Plate III. Figs. 33-25 ; Plate IV. Figs. 37-33 ; Plate V. Figs. 33-36. 



Figure 23 (Plate TIL) shows a latei-al view of an egg at the completion 

 of the first mitotic phase. The quartet 5.1 has divided, forming 6.1 and 

 6.2. Applying the test as in previous generations, it is readily seen that 

 this is a right spiral. The remnants of the nuclear spindle and the 

 asters leave no question as to the relationship of the cells in this egg. 

 The upper derivative lies to the right of the lower one in every case. 

 Eeference to the quartets 6.1 and 6.2 in Figures 27-29, 31-34, 36, of 

 four other eggs, shows the constancy of the direction of this spiral. The 

 division in this quartet is about equal, and is synchronous in the four 

 quadrants. 



