CYTOKINESIS. 87 



probably in the groove between the germ halves {cj. figs. 76, 92, 93). As soon as 

 the nuclear membrane is broken the entire mitotic figure moves out from the animal 

 pole towards the periphery of the cell ; during this movement the sphere substance 

 remains in its former position close to the animal pole. This peripheral movement 

 of the spindle continues until its outer pole comes almost into contact with the cell 

 wall, while the inner pole lies nearer the middle of the cell, fig. 94, text figs. XXVI, 

 XXVII. This position of the spindle is a perfectly definite one ; the outer pole 

 of the spindle lies near the middle of the oblique wall between the first and second 

 quartettes, and the spindle axis is parallel with the wall between contiguous cells 

 of the first quartette, figs. 94, 95. Although the spindle nearly doubles in length 

 from the metaphase to the late anaphase, the inner pole remains fixed in position, 

 Avhile the outer pole is pushed further and further into the pointed extremity of 

 the cell, fig. 95. During this elongation of the spindle the cell body also elon- 

 gates in the same axis, and the equatorial constriction appears and cuts off a 

 small peripheral cell from a large apical one ; the small cells are the irocJwblasis, 

 the large ones the cephaloblasts. One of the first quartette cells (Id) is slightly 

 smaller than the other three, fig. 94 ; it divides a little later than the others, and 

 the cephaloblast to which it gives rise is smaller than the others, though the trocho- 

 blasts are all of the same size, fig. 96. This cleavage is a particularly interesting 

 one since it i-epresents a very unequal division of an apparently homogeneous cell. 

 In this case the eccentricity of the spindle would appear to be the immediate cause 

 of the unequal cleavage ; this eccentricity is the result of movements in the cell, 

 which begin coincidently with the breaking of the nuclear membrane. 



This cleavage is a lasotropic one, and in the telophase the contents of the cepha- 

 loblasts rotote in a hieotropic direction, while those of the trochoblasts rotate in a 

 dexiotropic direction, the middle of the spindle in each cell being carried to the left, 

 text fig. XIV. In the cephaloblasts the nuclei centrosomes and spheres lie nearer 

 the middle of these cells than in the preceding rest stage. 



In this position the centrosomes and central spindles for the second division 

 of the first quartette cells arise, as in all the preceding cleavages. The spindles 

 and cell bodies elongate, and the equatorial constrictions occur as is usual. This 

 cleavage is dexiotropic, the spindle axes being at right angles to those of the 

 preceding cleavage {cf. figs. 95 and 97). It is also unequal, but in this case the 

 outer products are larger than the inner ones, whereas the reverse was the case in 

 the preceding cleavage. These larger outer products are the basal cells in the arms 

 of the ectodermal cross, while the inner ones are the apical cells. One of the cephal- 

 oblasts. Id (fig. 97) is smaller than the other three, and its division is not so unequal 

 as that of the others. Accordingly, the basal cell in the arm of the cross in this 

 quadrant, Id^^ (figs. 98, 99), is not so large as the other basal cells ; all the apical 

 cells are approximately equal in size. The sphere substance left in the cephalo- 

 blasts, at the close of the preceding cleavage, passes at this division into the apicals, 

 where it rapidly disintegrates, fig. 97. 



The inequality of this cleavage can scarcely be due to the eccentricity of the 



