No. 2.] DEVELOPMENT OF MARINE ANNELIDS. 295 



without the slightest regard to this rule of alternating cleavage. 

 Their only concern seems to be to form a pattern bilaterally sym- 

 metrical with respect to the sagittal plane of the embryo. This 

 tendency is well illustrated in the formation of the cross in all 

 forms where the latter occurs; in the characteristic first divi- 

 sion of the mesoderm cell (</"), even in Discocoelis; and in the 

 further division of the somatic-plate cells (p. 241). These cleav- 

 age phenomena present three general features which are to 

 me especially interesting: (i) certain cells divide bilater- 

 ally, while their adjacent sister cells continue to divide in the 

 alternating oblique rhythm; (2) the former cells suddenly dis- 

 engaged themselves from the influence, whatever it may be, 

 which compels the parent cells and sister cells to divide in 

 the alternating rhythm; (3) in most cases the cells which so 

 promptly respond to the influence of bilaterality, so to speak, 

 are the anlagen of bilaterally symmetrical structures, e.g., 

 mucous glands [Amphitrite), head-kidneys {Nereis), mesoderm, 

 somatic plate, etc. 



The direction of the first division of the paired mesoblast 

 cells in Amphitrite stands in direct contradiction to the "law" 

 that the spindle lies in the direction of least pressure or that it 

 lies in the long axis of the protoplasm of the cell, for it coin- 

 cides with the direction of greatest pressure, and is at right 

 angles to the long axis (cf. p. 247). Additional importance 

 attaches to this division, for, from the point of view of 

 cell homology, it is a reminiscence of a surface division 

 which still persists in some forms ; and thus we have here a 

 contest between the influence of mechanical surroundings and 

 an intrinsic tendency to divide in a certain direction, at a certain 

 time, and the latter prevails. 



The "parent cross-cells" in Chcstopterus, on the one hand, 

 and in Nereis, Lepidonotus, Amphitrite, etc., on the other, afford 

 remarkable examples of cells of similar origin and position 

 dividing in different animals in fundamentally different direc- 



division in the above forms. Why these divisions are retarded, and what 

 is the destiny of the cells, are, unfortunately, unsolved problems. It looks, 

 however, as though the direction of the cleavage was influenced by the position of 

 the surrounding cells, and, therefore, may accord fairly well with the mechanical 

 hypothesis. 



