No. 3.] THE CELL-LINEAGE OF NEREIS. 453 



We come now to the main point, which is that the rotation of 

 the cells is in the spiral type in many cases predetermined in the 

 parent-cells, as is proved by the position of the spindles and by the 

 form of division. It is, in other words, to a greater or less extent, 

 a virtual instead of an actual rotation. This is the case, for ex- 

 ample, in Asterina, as described by Ludwig, where the division- 

 planes of the two primary blastomeres are at right angles to 

 each other at the second cleavage. The same is true of the 

 polyclade embryo (Lang). It remains to be seen how far the 

 virtual rotation is accompanied by an actual rotation of the cells 

 after their formation. In the later cleavages of Nereis {e.g. in 

 the formation of the rosette-cells at the fifth cleavage and of the 

 intermediate girdle-cells at the fourth), the spindles lie in the un- 

 divided parent-blastomeres, exactly vertical to the future plane of 

 division and the rotation is purely virtual. In the third cleavage, 

 however, this is not the case, since the obliquity of the spindles is 

 at first very slight and constantly increases during the division, 

 until they form an angle of nearly 45° with the vertical axis. 

 There is also in this case a certain amount of actual rotation 

 after the complete separation of the cells, as may be seen by a 

 comparison of Figs. 11 and 12. Regarding the second cleavage, 

 the available data are very scanty, since this stage has not been 

 accurately examined with reference to the point. I can state 

 positively, however, that in Ascaris megalocephala (which, 

 through the courtesy of Dr. Boveri, I have been enabled to 

 study in the living state as well as in preparations) there is 

 a very extensive actual rotation in the four-celled stage, after 

 the full completion of the division. The value of this case as 

 an illustration is, however, lessened by the fact that the second 

 cleavage differs radically from the usual form. 



To sum up, I conclude that the spiral form of cleavage is 

 owing to a precocious appearance of the alternation of the cells, 

 which, in its turn, is a result of mutual pressure. The "true 

 radial" type differs from the spiral only in the fact that the 

 alternation appears at a later period ; in other words, the cleav- 

 age longer adheres to the primary type. The primary type owes 

 its characteristics to the form of the ovum, in accordance with 

 the general laws of cell-division. Thus the characteristics of 

 the spiral period are, in their broadest outlines, the result of 

 mechanical conditions which have no relation to the adult 

 structure. 



