36 bulletin: museum of comparative zoology. 



The other thirty cells show a regular arrangement. The four quadrants 

 may be distinguished as at the beginning of the fifth cleavage, each 

 quadrant now containing eight cells^ showing a characteristic arrauge- 

 nieut. From ventral to dorsal we may now distinguish five layers. The 

 first three layers contain each eight cells, two from each quadrant. In 

 the quadrants A, B, and C, the two cells of a given layer are equal 

 (Plate 6, Fig. 47), In quadrant D there is great irregularity. The 

 two cells of the ventral layer are extraordinarily unequal, constituting 

 the large partly interior cell d^'^, and the minute vesicle d^-'^, also 

 enclosed within the other cells (Plate 5, Fig. 38). In the second 

 and third layers the two cells are likewise unequal, though less 

 markedly so. 



The fourth layer consists of a single large cell from each quadrant; 

 that of quadrant D being the largest (Plate 6, Fig. 45). 



The fifth layer consists of four small cells at the dorsal pole of the 

 ^eo (Fig. 4.5). The arrangement at the animal pole formed at the four- 

 cell stage (Fig. 8) is still maintained. The quadrants B and D are in 

 contact for a considerable distance, whereas A and C do not touch. In 

 one of the points where three cells of difierent quadrants meet (in this 

 case b^-^y c^-^, and d^-^) lies the polar cell. 



The first cells in which indications of cleavage are observed are again 

 the large cells of the D quadrant, d^-^ and d^-^. Spindles are formed in 

 these at about the same time (Plate 6, Fig. 48). The processes taking 

 place in the two cells differ, and must be considered separately. 



In c?®\ after the giving off of the small vesicle d^--, the nucleus very 

 quickly enlarges to its original size. The aster begins to elongate at 

 right angles to the position of the previous spindle (Plate 5, Fig. 38). 

 But at almost the same time a rotation takes place, and by the time 

 the two asters are fully separated the line connecting them is seen to 

 be nearly antero-posterior (Plate 5, Fig. 42), The movement continues 

 until the axis of the complex becomes exactly antero-posterior, and a 

 spindle is formed in precisely the same position as the spindle for the pre- 

 ceding cleavage. This spindle is shown in the sagittal section, Figure 

 48. Its anterior end lies just ventrad of the small vesicle formed at 

 the previous cleavage. Division now takes place, and a second small 

 vesicle is given off to the point in the median plane lying just ventrad 

 of the vesicle formed at the fifth cleavage. Figure 49 shows the pro- 

 cess of formation of this vesicle, and Figure 50 shows the condition of 

 affairs after the division is finished. In later stages the two vesicles are 

 visible, lying beneath the cells of quadrants A and B, in the place 



