EMBRYOLOGY OF THALASSEMA MELLITA 183 



Soon after, the lower sinistral cell {2a...., etc.) buds off, with the 

 spindle lying in the same direction, a small cell (2(^o 2. 2) ^tc.) 

 toward the fourth quartet (PL II, Figs. 16 and 18). Accord- 

 ingly, at this stage (about 80 cells) there are five cells in each 

 quadrant w^ith exactly the same arrangement, and this radial 

 symmetry is retained, almost perfectly, until the beginning of 

 gastrulation. The divisions of the sinistral cells may be fol- 

 lowed in the figures (PI. II, Figs. 18, 19, 20, 21). The upper 

 dextral cell in each quadrant (2rt'i 1, etc.) soon divides dexio- 

 tropically and is follow^ed after a time by the lower dextral 

 (2^1 o, etc.) in the same direction (PI. II, Figs. 18 and 21). The 

 three upper cells in each quadrant (2(^j ;, 1, 2a^^ ^t -^i.-i.-i) later 

 become ciliated and form a part of the secondary prototroch (PL 

 II, Fig. 24). The lower cells in the a and c quadrants (2<?i o 2, 

 2^1. 2. 2) giv^ I'ise to a part of the lateral post-trochal ectoderm 

 and in the d quadrant (2^^ ,_, o) to a part of the stomodaeum. The 

 origin and fate of the " oesophagoblasts " (Eisig, '98) will be 

 described in the next section. 



c. Third Quartet. — Especial interest attaches to the third 

 quartet in Thalassona, since from it arises the greater part of 

 the functional ectomesoblast. At the 64-cell stage each member 

 of the third quartet had divided once leiotropically and un- 

 equally (PL I, Figs. 10 and 11). The low^er larger cell (3^2* 

 etc.) next divides dexiotropically and unequally in each quad- 

 rant (PL II, Figs. 13, 16 and 17). The smaller product of this 

 division is dextral in the a, b and d quadrants, but in the c 

 quadrant is sinistral. Accordingly, in the c and d quadrants 

 the larger cells lie on each side of 4</ and the smaller cells lat- 

 erally (PI. II, Fig. 13). These, as in Podarke, are the first bi- 

 lateral divisions in the post-trochal region. The two larger 

 cells, ^'2.1 ^^^ 3'^2. 1 show^ a marked tendency to sink in (PL II, 

 Fig. 18), but come to the surface to bud off two more small 

 cells (PL II, Figs. 19 and 20). When the embryo consists of 

 about 130 cells they are seen lying in the cleavage-cavity en- 

 tirely below the surface (PL II, Fig. 21). Their cell-lineage ac- 

 cordingly is 3^2.1.2.1 a^d 3<'/2. 2. 2. 1- The remarkable similarity in 

 the origin of this part of the ectomesoblast with that of Podarke 

 is discussed beyond. 



