No. 3-] THE CELL-LINEAGE OF NEREIS. 409 



sixth, as already explained). The movements of the embryo 

 now render this impossible, and hence the later changes have 

 been studied mainly from preparations, though most of the 

 stages have also been observed in the living state. At the tenth 

 division (the result of which is shown in Fig. 61) each of the 

 teloblasts again divides transversely into two unequal parts, 

 the smaller, as before, lying in front. I do not feel certain, 

 however, that this division really takes place {i.e. I may 

 have confounded it with the last), as the cells of the ventral 

 plate are now so numerous that it is impossible to trace the 

 exact genetic relationships among them. The occurrence of 

 such a division is an inference from the fact that the teloblasts 

 are found dividing in this manner a considerable time {i.e. from 

 one to two hours) after the stage shown in Fig. 59, and the cells 

 of the ventral plate are more numerous than before. The point 

 (which is mainly of importance in a theoretical consideration of 

 the history of the nuclear figures) could easily be determined by 

 a re-examination of these stages. 



The eleventh division (the result shown in Figs. 6'i^, 64) is 

 one of great importance, because by it the outer pair of telo- 

 blasts disappears. Each divides lengthwise into nearly equal 

 parts, which can scarcely be distinguished from the remaining 

 cells of the ventral plate. I have never succeeded in finding a 

 specimen to show the spindles of this division, and hence only 

 infer its occurrence from the position of the cells, but I have no 

 doubt of its occurrence. The two remaining teloblasts will 

 henceforward be called the residual teloblasts. 



The twelfth division (Figs. 65, 66) marks another important 

 step ; viz. the diver'gence of the residual teloblasts. Each divides 

 into unequal parts in the same manner as in the eighth division, 

 and the two teloblasts at the same time move slightly apart. 

 The interspace is occupied by one or two clear, very thin cells, 

 the position of which is clearly shown in the figures, and which 

 I shall describe hereafter. Shortly after this division the prod- 

 ucts of the two outer teloblasts also divide, one of them trans- 

 versely, the other longitudinally. In the very perfect preparation 

 figured in Figs. 66 and 'j6 the four spindles were perfectly 

 symmetrical. 



Later divisions. Beyond this point I cannot certainly enu- 

 merate the divisions, though I have little doubt that Figs. 6^^ 



