Manchester Memoirs, Vol. I. (1906), No. 3. 73 



handedly, a third quartette (3(T, etc.) right-handedly, and 

 a fourth (4^, etc.) left-handedly again. The micromeres 

 themselves obey the same law of alternating direction 

 of cleavage in successive divisions. These four quartettes 

 and the residual macromeres contain the material for 

 the germinal layers. In one case {TetJiysf'^ a fifth 

 quartette of micromeres is produced. 



Let us now follow, first, the fate of these cells in 

 different cases. 



The first quartette becomes the ectoderm of the pre- 

 oral region, including the apical organ, when present, the 

 cerebral ganglia, and the whole or a part of the prototroch. 



In the second quartette the posterior blastomere, 2d, 

 is often very large, and known as the first somatoblast ; 

 it gives rise to the ventral plate of ectoderm. The cells in 

 the remaining quadrants {2a, 2b, 2c) usually also give rise 

 to ectoderm, forming sometimes the stomodaeum or 

 oesophagus {Capitella)^^ sometimes contributing cells 

 to the prototroch. There are, however, many cases in 

 which mesoderm also arises from this group of cells ; in 

 Crepidula ^* derivatives of 2a, 2b, 2c produce larval 

 mesoblast, in Unto-'" a descendant of 2a, in Aricia"^^ 

 possibly 2c and 2d ; in Aplysia depilans (according to 

 Georgevitch^^) 2c and 2d are the primary mesoblasts, and 

 in Dreissensia"-^ mesenchyme is derived from the descend- 

 ants of all four cells ; while in the Turbellaria, according to 

 Hallez^® and Lang''", the blastomeres of this quartette are 



'^^ Viguier, C, Arch. Zool. Exp. ser. 3, vol. 6, p. 37—58, 1898. 



*» Eisig, H., Mitt, Zool. Stat. Neapel, vol. 13, p. 1—267, 1899. 



2* ConUlin, E. G.,Journ. Morph. vol. 13, p. I — 204, 1897. 



*^ Lillie, ¥.,Journ. Morph., vol. lO, p. I— 84, 1895. 



'^o Wilson, E. B., Biol. Led. Woods Holl, 1898, p. 21—42. 



2'^ Anat. Anz., vol. 18, p. 145 — 174, 1900. 



^^ Meisenheimer, J., Zietschr. wiss. Zool., vol. 69, p. I — 137, igoi. 



^' " Contributions a I'histoire naturelle ties Turbellaries." Lille, 1879. 



** "Die Polycladen des Golfes von Neapel. Leipzig, 1884. 



