452 R. M. PACE. 



divides the egg into two symmetrical halves^ each containing 

 equal quantities of yolk : it is completed in about twenty 

 minutes. This division is followed by another, in a plane at 

 right angles to the first, dividing the egg into four spheres, 

 which are to all appearance equal in all respects (PI. 23, 

 fig. 43; PI. 24, fig. 52). 



To simplify the following account of the cell-lineage of 

 Flustrella, these four first-formed cells have been dis- 

 tinguished in the figures by the letters A, B, C, and D. Cells 

 arising from these are denoted by the letter of the particular 

 cell from which they have been derived, with the addition of 

 a negative index to indicate the generation to wliich the cell 

 belongs, and of a positive index to denote the number of cells 

 in that generation at the moment of the formation of any 

 given cell, and the order of their formation : thus A^ denotes 

 the fourth cell derived from A in the fifth genei'ation.^ 



The 8-cell Stage. — The four cells A, B, C, and D, again 

 dividing in a plane at right angles to each of the former 

 divisions, give rise to eight cells : of these, the four lower 

 cells are larger, and contain more yolk than the four upper 

 ones (PI; 23, fig. 44). 



The polar bodies are seen in the living egg to lie on the 

 surface of the smaller cells, and these smaller cells and their 

 derivatives always lie on the upper surface of the egg, wliich 

 later on becomes the dorsal or aboral surface of the Inrva. 



The 12-cell Stage. — The four small upper cells next 

 divide (PI. 23, figs. 45 a-h) each into two unequal cells by 

 a cleavage lying at an angle of 45° to the primitive cleavage 

 plane. The ring of small cells which is thus formed becomes 

 shortly afterwards rearranged into two rows of four cells 

 each, so that the embryo now consists of two parallel series 

 of small cells overlying the four large yolk-laden ones. 



Owing to this new arrangement of the embryonic cells, it is 



^ 111 view uf the use whicli has been made of a somewhat similar system of 

 notation by otiier authors, it should be stated that the symbols here used have 

 been adopted only for the sake of clearness, and that they liave no reference 

 to any of the theories of cell genesis which have been put forward. 



