No. 2.] DEVELOPMENT OF MARINE ANNELIDS. 237 



Summary. — The fertilized egg of Ai7ip/iitriie is spherical, 

 about loo/x. in diameter, and covered with a wrinkled membrane. 

 The first cleavage is unequal, and in the 4-cell stage one 

 blastomere is larger than any of the others. A vertical plane 

 passed through the larger blastomere and the one diagonally 

 opposite corresponds to the sagittal plane of the future embryo. 

 The number of cells increases in geometrical progression from 

 one to sixty-four. From the 2 to the 64-cell stage every cleavage 

 furrow is oblique to the meridian of the egg, and the direction 

 of the obliquity regularly alternates with each succeeding cleav- 

 age. From the 8-cell stage onward we distinguish an anterior 

 or upper hemisphere — the four upper cells or their descend- 

 ants, and a posterior or lower hemisphere — the four lower 

 cells or their descendants. In consequence of a slight rotation 

 of one hemisphere upon the other, the second cleavage furrow 

 more nearly coincides with the future sagittal plane in the upper 

 than in the lower hemisphere. In the 64-cell stage the material 

 for the several germ layers is completely sorted out so that one 

 cell represents the future mesoderm, seven cells the entoderm, 

 and the remaining fifty-six cells, the ectoderm. The latter fall 

 naturally into groups, which from the animal pole are: (i) 

 rosette, (2) cross, (3) intermediate cells, (4) primary prototroch, 

 (5) secondary prototroch, (6) somatic-plate, (7) the rest of the 

 ectoderm of the lower hemisphere. Then come the mesoderm 

 and entoderm. The regularity of the cleavage ceases abruptly 

 at the 64-cell stage, so that from this time one can best follow 

 the cells in groups. 



II. Later Cleavage to Formation of Paratroch. 

 a. Anterior hemisphere. 



By the time the 64-cell stage is actually attained, the parent 

 cells of the cross, a!"^, h"'^, c'■^ ^'■^ contain karyokinetic spindles 

 and soon divide, sometimes simultaneously, but often with slight 

 differences in time (Fig. 24). The furrows always cut the 

 meridian of the egg at right angles. There is never a trace of 

 the oblique cleavage characteristic of all the previous divisions. 

 The resulting cells form the pattern of a cross, with arms in- 



