No. 4-] CELL LINEAGE OF POD ARK E OBSCURA. 2OI 



cross furrow seems not to retain its original position, and, 

 although the small cell x 1 ' 2 appears, other divisions of the 

 second group of micromeres occur before any other landmarks 

 are to be seen, and orientation is difficult. A fifth group of 

 micromeres is given off, and the regular invaginating plate of 

 eleven cells appears. One member of the fourth group of mi- 

 cromeres divides bilaterally at the surface, forming what I 

 suppose is the mesoblast. The rosette is formed as in other 

 cases, but the cross remains radially symmetrical until a late 

 stage in the segmentation. 



The primary trochoblasts arise as in Podarke, but do not 

 divide again, thus forming two in each quadrant instead of 

 four. Wilson 1 showed that the prototroch of Hydroides is 

 composed of eight cells. 



From these observations it is evident that equal cleavage in 

 annelids is not in any way caused by or an expression of a 

 lack of differentiation in the ovum. Although the dorsal can- 

 not be distinguished from the ventral quadrant until the 64-cell 

 stage, it is perfectly possible at the 4-cell stage to determine 

 by means of the cross furrow the median plane of the future 

 embryo. That this cross furrow sometimes varies in direction 

 in later stages is fully explained by the fact that the cells form- 

 ing it are very small (after the fourth group of micromeres are 

 given off), and more or less of what I have called "pseudo- 

 invagination " occurs. The furrow itself is short as compared 

 with such forms as Nereis or Amphitrite. Under these con- 

 ditions, as Conklin 4 has shown, the direction of the cross 

 furrow may easily vary because of variations in pressure on 

 the entoderm cells. 



Further, the regular alternation from a right to a left-handed 

 cleavage, the regular appearance of certain definite cells at 

 certain definite times, indicate that we have here a differen- 

 tiation as complete as in any form with unequal cleavage. At 

 the 32-cell stage the embryo is approximately spherical, and is 

 surrounded by a thin, much-wrinkled egg membrane. The 

 four cells at the animal pole are a very little larger than those 



i E. B. Wilson, /. c., p. 398. 



