1 8 LILLIE. [Vol. X. 



" the same thing is repeated and results in the formation of a 

 cap of small cells lying on a large one." The large cell is held 

 to be entodermal always, but in Unio I am certain that it is 

 not so. 



It seems impossible at present to reduce the two hitherto 

 described forms of cleavage in lamellibranchs to a common 

 law; but Korschelt and Heider compare Rabl's account of the 

 cleavage of Unio with Hatschek's account of Teredo, in spite 

 of the fact that the first cleavage of the larger cell is meridional 

 in the first case and equatorial in the second, according to the 

 description given. According to Ziegler and Stauffacher, the 

 cleavage of Cyclas is like that of Teredo ; the latter's sections 

 are convincing ; one can no longer doubt that the first cleavage 

 of the larger cell is equatorial in this form. How this type of 

 cleavage can be derived from the oblique type, it would be 

 difftcult to say. But perhaps the only difference is that the 

 obliquity of the spindle is greater. 



3. From the Eight to the Seventeen-cell Stage. 



A nine-cell stage is formed in Unio complanata. This stage 

 is reached from the eight-cell stage by a division of the poste- 

 rior macromere D in an equatorial plane (PI. II, Figs. 17 and 

 18). The division is unequal, the ectomere d^ being much 

 larger than the macromere^ D. d^ is the "first somatoblast " 

 (v. Wistinghausen) and in succeeding stages will be designated 

 by the letter X. It is also the first of the second generation 

 of ectomeres. The formation of the other members of the 

 second generation of ectomeres is illustrated in Fig. 19, PI. II 

 (a view from the right side) ; a}, b^, and c^ are larger than 

 their parent macromeres A, B, C (PL II, Figs. 20 and 21). 

 The cleavage-spindles of the second generation of ectomeres 

 are leiotropic. 



d^ divides next, thus producing the thirteen-cell stage (PI. 

 II, Fig. 20, apical pole). The divisions of the other members 

 of the first generation of ectomeres soon follow. The order of 



^ The term " macromere " will be retained throughout for the four hypotropic 

 cells (Goette) without reference to their relative size. 



