Selected Invertebrates 



319 



{la^-ld}) and four lower cells {la--l(P)-^ see 

 Figure 120L>. If la^ is isolated (Fig. 120E), 

 it develops into two secondary trochoblasts, 

 several non-ciliated cells and a small number 

 of apical tuft cells, whereas an isolated la^ 



are isolated or remain as part of the whole 

 egg. Each cell or group of cells differentiates 

 in a specific way because specific portions 

 of a heterogeneous cytoplasm are segregated 

 into them. 



Fig. 120. Specification of blastomeres in molluscan eggs. {A, C, E and F from Wilson, '04b; G, H and / 

 from Wilson, '04a.) A, Normal trochophore larva of the moUusk Patella. B, Generalized scheme of cleavage 

 in annelids and mollusks (from Wilson, 1899). The first somatoblast (2d) is indicated by sparse stippling; 

 the second somatoblast (4d) is indicated by heavy, dense stippling. C, Differentiation of isolated cell of first 

 quartette of micromeres of Patella. D, Generalized scheme of cleavage of first quartette of micromeres. 

 E, Differentiation of isolated la^ cell of Patella. F, Differentiation of isolated la^ cell of Patella. G, Normal 

 trochophore larva of the mollusk Dentalium. H, Type of larva which differentiates from the Dentalium egg 

 lacking first polar lobe, or from the following cells when isolated: AB, A, B, C, la, lb or Ic. Note absence of 

 apical tuft and post-trochal region. /, Type of larva which differentiates from the Dentalium egg lacking only 

 the second polar lobe, or from an isolated Id cell. Note that only the post-trochal region is lacking. 



blastomere (Fig. 120F) divides only twice to 

 produce four primary trochoblasts; etc. Es- 

 sentially the same results were obtained with 

 the Nereis egg except for minor differences 

 which correspond to slight differences in 

 cell lineage in these two animals. Thus with 

 each successive cleavage, cells become more 

 sharply specified for one fate only, and that 

 fate is realized whether the cells involved 



Experiments on Eggs Possessing Polar Lobes. 



In the eggs of some annelids (Sabellaria, 

 Chaetopterus) and mollusks (Dentalium, 

 Ilyanassa) a pecvdiar phenomenon known as 

 polar lobe formation occurs, the first polar 

 lobe appearing just before the first cleavage 

 furrow, the second polar lobe appearing just 

 as the second cleavage is initiated. The cyto- 

 plasmic contents of the polar lobes enter the 



