No. 2.] DEVELOPMENT OF MARINE ANNELIDS. 



233 



veTiuaL 



TiSht 



ous role in the development. Neither of the cleavage furrows 

 coincides with the sagittal plane of the future embryo ; this 

 plane passes through the cells B and D (cf. Wilson,^ p, 3 86). 



All four cells undergo an almost synchronous right oblique 

 cleavage, and an 8-cell stage results (Figs. 5, 6, 7). In con- 

 sequence of the obliquity of the cleavage, the upper cells 

 a^, b^, c^, d^ are not directly superimposed on the large ones 

 ^„ B^, C^, Z>i, but alternate with them. For convenience in 

 description the cells a^, b^, c„ <^,, or their descendants, will be 

 regarded as constituting the upper 

 or anterior hemisphere. 



The relative position of the up- 

 per and the lower quartette ol 

 cells is significant, for it alters 

 the direction of the second cleav- 

 age furrow, twisting it so that its 

 direction on the upper hemisphere 

 is different from that on the lower, 

 — a relation which is maintained 

 throughout the subsequent cleav- 

 age. Each of the four cells has 

 divided unequally, but with a re- 

 markable result ; on the posterior hemisphere in the 8-cell stage, 

 Z>, is larger than A^, B^, C^, which latter are about equal to one 

 another in size, while on the anterior hemisphere, d^ and c^ are 

 equal in size, but larger than a^ and bi. The four original 

 cells have undergone a sort of compensating division, which 

 establishes upon the anterior hemisphere a bilaterally sym- 

 metrical group of cells, whose plane of symmetry nearly coin- 

 cides with the second cleavage furrow (Fig. 7). 



The polar globules remain where they were formed, and lie near 

 the meeting-point of the four upper blastomeres. There is already 

 a small segmentation cavity (Fig. 7). The yolk is more abundant 

 in the four lower cells, but the ^gg follows the same course of 

 development whether in one position or another, and often has 

 been observed to develop with the vegetative pole uppermost. 



8-16 cells. — Except for slight variation, the eight cells 

 cleave synchronously, and always obliquely to the left (Figs. 



Fig. II.— -Diagram of 4- cell stage, a, ani- 

 mal pole ; b, vegetative pole ; A, B, C, 

 and D, first four blastomeres, the an- 

 lagen respectively of the left, ventral, 

 right, and dorsal quadrants of the troch- 

 ophore. 



