KOFOID: DEVKLOPMKNT OF Ll.MAX. 51 



meeting in a point at the animal pole, wliile the ventral cross furrow is 

 correspondingly longer. In Umbrella (lieymons '1)3) the dorsal and 

 ventral furrows of this stage are parallel, i. e. are formed between the same 

 cells B and D, the cells A and C being considerably separated. This is 

 undoubtedly due to the presence of a large amount of yolk in the four 

 bliistomeres. Likewise in Planorbis (Kabl '79) and Neritina (Blochraanu 

 '81) we find tlie dorsal and ventral furrows of tliis stage similar to those 

 of Umbrella, rather than Limax. These cross furrows are an invaluable 

 aid in the determination of the axes of the later stages ; the question of 

 their relations and constancy will be discussed later. 



Fourth Generation. Third Cleavage. Eight Cells. 

 Plate I. Figs. 8-13 ; Plate II. Figs. 17-19; Plate III. Figs. 30, 31. 



About two hours and a half intervene between the beginning of the 

 four cell stage and that of the eight-cell stage. The third cleavage is 

 accomplished by the division of the quartet of the third generation, A, B, 

 C, D, into two superposed quartets (cf. Figure B, p. 52), A*^ — Z*", 

 and a*"^ — d*-^. The series of stages shown in Figures 8-13 (Plate I.) 

 represents the egg during this process. Figures 17 and 18 (Plate II.) 

 give apical and lateral views respectively of an eg'g with the spindles of 

 this generation. It will be noticed (Fig. 17) that the spindles in no 

 case stand vertically, but that they are inclined toward the right (right 

 and left being used as resident in the egg ; see my earlier paper, '94, 

 p. 180). The division of the chromatin elements in the spindles has 

 just begun, and there is no trace of a constriction of the cytoplasm. A 

 comparison of Figures 17 and 19 shows that the degree of obliquity of 

 the spindles has increased during the interval between the two stages. 

 Figure 19 i-epreseuts a stage in which the constriction of the upper from 

 the lower quartet, i. e. of the microraeres from the macromeres, has just 

 been completed. The obliquity increases during the division, so that at 

 its close the cells of the upper quartet lie in the furrows between the 

 cells of the lower quartet. Thus it will be seen that this apparent 

 shifting of the upper quartet upon the lower, known as the "spiral," 

 takes place in large measui'e during the division of the cells. It will 

 also be noted that the plane of division is at riglit angles to the axis 

 of the spindle. 



If we view the egg from the animal pole and pass from the lower 

 derivative of a pair to the upper, we move in the direction of the hands 

 of a clock, and therefore the division of this, the fourth generation, takes 



