3/8 WILSON. [Vol. VI . 



material is, as it were, sifted out and arranged ; but it is a very 

 remarkable and interesting fact that the cell-divisions by which 

 this is effected do not take place bilaterally in accordance with 

 the adult structure, but show a peculiar modification of radial 

 symmetry which is best characterized as spiral in character, and 

 which cannot be reduced to the bilateral type. 



The cleavage is total and unequal ; no cleavage cavity is 

 formed, the gastrulation is strictly epibolic, and the "blasto- 

 pore " closes at a point that nearly coincides with the lower 

 pole, i.e. 1 80° from the point at which the polar cells are formed. 

 The first cleavage is transverse to the median plane of the tro- 

 chophore and at right angles to the plane of the prototroch, 

 hence horizontal with respect to the adult body, owing to a 

 peculiar shifting of the axes, which will be described in the 

 course of the paper. The second cleavage-plane ultimately 

 coincides with the median plane of the trochophore and of the 

 adult body, thojigh it does not divide the egg into eqnal halves. 

 The third cleavage-plane is as usual horizontal or equatorial 

 (parallel to the prototroch) and separates four smaller micro- 

 meres above from four macromeres below. 



Three sets of four "micromeres" each are successively sep- 

 arated from the macromeres {A, B, C, D, Diagram i) by the 

 TT following invariable law. The first 



y^^ "^i^^^"*^— ><v^^\ ^°^^ (^"^' ^^^ ^^> '^^) ^^^ formed in 



/ /7\ ■: {V\A, a right-handed spiral, the second 



I (S-^Vt i I V\-^ I ^^^^ ^'^^' ^^' '^^' ^^^ ^^ ^ left-handed 



« V A ...i,^.J,j ._ } k___ y I spiral, and the third set (a^, b^, c^, 



f\t T'^-Tx^* 7v\ ^^^ ^^ ^ right-handed spiral like 



\\\ mA \ '! / — \ I ^^^ ^''^^ ^^^* ^ follow Lang's ter- 



y y S^^__,x^4— ^ ^ J^ I minology, as applied to the ovum 



\ ^~i* 1^-X * J / ^^ ^^^ polyclade, Discocoelis, with 



\^.>/^[--— —J-Y"''^ y which the segmentation of Nereis 



^vis^^y^^^^^'^ — -^ accurately agrees up to a late 



H stage.) From these twelve micro- 



DiAGRAM I. meres the entire ectoblast arises. 



Their formation is followed by the separation of the primary 



mesoblast, or " second somatoblast " (d^ or M) from the left 



posterior macromere, with which event the differentiation of 



the germ-layers is completed and the strictly spiral period 



ends. The micromere d"^ or X, which is much larger than the 



