CYTOKINESIS. 85 



daughter centrosomes and central spindles again arise from the mother centro- 

 somes and these initial spindles stretch across the outer and upper side of each 

 nucleus in the groove between the germ halves as in the preceding cleavages. The 

 dexiotropic movement of the substance of the macromeres continues until the right 

 pole of each spindle is brought close to. the right side of each macromere and into 

 the angle between two adjacent cells of the second quartette. Here the sphere 

 material is spread in a ring as in the preceding cleavages, and a lobe of cytoplasm 

 is formed over the upper pole of each spindle, text fig. XXXI. These lobes are 

 then constricted from the macromeres, thus forming the third quartette of micro- 

 meres, fig. 96. Here, as in the preceding cleavages, the whole of the sphere sub- 

 stance left in the macromeres goes into the upper cell products, i. e., in this case, 

 into the third quartette. In the telophase the contents of the third quartette cells 

 rotate in a dexiotropic direction, while those of the macromeres rotate in a lajotropic 

 direction (cf. figs. 96, 97, 98) ; the extent of these rotations, however, is not so 

 great as in the preceding cleavages. During these movements in the telophase 

 the centrosomes and spheres never move under the cells lying nearer the animal 

 pole, but alwaj-s remain at the margin of these cells and in contact with the free 

 surface of the cells in which they lie, figs. 97, 98 and text figs. XV, XVI. 



At the sixth cleavage one of the macromeres, D, divides much earlier than the 

 others, and gives rise to the mesentoblast cell, 4d, figs. 97, 98 and text fig. XVI. 

 The centrosomes and central spindle arise from the mother centrosome on the 

 outer and upper side of the nucleus of macromere D, and it is probable that the 

 central spindle lies in the groove between the germ halves, since this groove is plainly 

 apparent in the daughter nuclei at the close of this cleavage (text fig. XVI), showing 

 that the germ halves have been divided, as in all the preceding cleavages. As soon 

 as the nuclear membrane has dissolved at the poles of this spindle the entire 

 macromere becomes rounded and stands out from the other macromeres, being in 

 ■ contact with them by relatively small pressure surfaces. The spindle then becomes 

 nearly vertical in the macromere, its upper pole being inclined slightly to the 

 left. The upper pole of the spindle lies near the surface while the lower pole 

 is near the middle of the macromere. Then a large lobe forms at the upper pole, 

 extending under the micromeres as far as the polar furrow. This lobe contains 

 not only all the cytoplasmic area and sphere substance of the blastomere D, but also 

 a large amount of 3<"olk, text fig. XVI. This lobe then constricts off from the macro- 

 mere, forming the mesentoblast cell, 4d. This cell is larger than any of the micro- 

 meres, and is so covered by them that I have been unable to observe all the move- 

 ments of its cell contents in the same satisfactory way which is possible in the 

 micromeres. It it certain, however, that its cytoplasmic portion, containing the 

 nucleus, centrosome and sphere, turns in a Inotropic direction until these parts are 

 carried under the micromeres and to that part of the cell which lies nearest the 

 animal pole. After this the mesentoblast divides equally into right and left halves 

 as shown in fig. 99, and at the close of this division the nuclei, centrosomes and 

 spheres again rotate through an angle of 90° until they come to lie in those portions 



