No. 2.] THE MESOBLAST-BANDS JN ANN EI JDS. 209 



I have followed the cleavage process many times and find it 

 to be extremely constant, the variations being so insignificant 

 as to have no effect on its general character. The first cleavage 

 divides the ovum into a larger {AB) and a smaller {CD) segment ; 

 the second divides CD into equal parts, C and Z>, and AB into 

 unequal parts, of which A is the largest of the four, and B is 

 intermediate in size between it and the two smaller segments, 

 C and D. The third or equatorial cleavage separates four 

 micromeres {a, b, c, d) at the upper pole from four macromeres 

 {A, B, C, D, respectively, Fig. i). The later development shows 



n 



Fig. 2. — View from the upper pole of the sixteen-cell stage of Nereis. X, the 

 first pro-teloblast which has been separated from A. 



that the second cleavage-plane (II.-II.) corresponds precisely to 

 the future median vertical plane of the adult body ; C and D are 

 the anterior macromeres ; A and B, the posterior. The embryo 

 is, therefore, not bilaterally symmetrical, from any point of view. 

 Bilateral symmetry is, however, soon to be established. 



At the fourth cleavage (Fig. 2) the four micromeres divide, 

 not quite equally, and three new micromeres are formed from 

 B, C, and D, respectively. A, at the same time, separates off 

 a large characteristically granulated cell {X), which I shall call 

 the first pro-teloblast. The regularity of the cleavage now 

 ceases. A little later A again divides, separating off a second 

 cell (F), somewhat smaller than X, but with a similar granu- 

 lation of the protoplasm. This I shall call the second pro- 



