No. 2.] DEVELOPMENT OF MARINE ANNELIDS. 235 



(cf. " Law of Alternating Spiral Cleavage," Wilson,^ Kofoid,* 

 etc.). If the cleavage were to continue in this regular fashion, 

 we should find each of the thirty-two cells dividing at about 

 the same time, and the cleavage furrow cutting each cell ob- 

 liquely to the left. The result would be sixty-four cells having 

 the regular arrangement characteristic of the earlier stages. 

 Amphitrite falls slightly short of the ideal, for some cells divide 

 sooner than others, and the most precocious are ready to divide 

 again, when the most tardy ones have divided once. This 

 tendency was recognized in the earlier cleavage, 16-32 cells, 

 and has now become accentuated (PI. XI, Figs. 17-28). Except 

 for this comparatively slight irregularity in time of division, 

 the cleavage of all thirty-two cells takes place with the rhyth- 

 mical regularity characteristic of the typical alternating oblique 

 cleavage : the cleavage of every cell is oblique and to the left. 



The transition from the 32 to the 64-cell stage is especially 

 interesting, because it is the last instance of the rhythmical 

 division of all the cells, and because by these divisions the 

 prospective germ layers become completely segregated into 

 special cells, and the primary prototroch differentiated. The 

 order of division can be seen by consulting Figs. 17-28. As a 

 rule, almost immediately after the completion of the 32-cell 

 stage, Z>3 divides into D^ and d'' (= M). The former is the 

 smaller and is pushed outward in a peculiar manner so that it 

 lies at first above the level of the other cells (Fig. 20), but later 

 crowds in among them, and forms part of the entoderm plate. 

 d* (= M) is the original mesodenn cell, and apparently gives 

 rise to all the mesoderm of the body. Before this division is 

 completed, the eight daughter cells of the primary trochoblasts 

 prepare to divide (Figs. 17, 21, 22, colored brown). All the 

 sixteen cells resulting from this cleavage acquire cilia, and con- 

 stitute the primary prototroch (Figs. 18, 19). Meanwhile the 

 four animal-pole cells divide almost simultaneously, but with 

 many slight variations in different eggs. The innermost prod- 

 ucts, «j.3, <^j.3, iTj.j, and (^1.3, constitute the '^ apical rosette,'' while 

 the outermost are parent cells of another characteristic pattern, 

 — the ''cross" a'■^ h"'^, c^'^, and d'"'^ (colored blue. Figs. 18, 24). 



The two daughter cells of the somatoblast, i.e., d'^'"' and (f" 



