1 2 LILLIE. [Vol. X. 



part of the embryo, which leads at last to fundamental dis- 

 turbances of the axial relationships of the parts. 



Figs. 9 and lo (PI. I) show two different phases of the four- 

 cell stage. In the earlier phase (Fig. 9) B and D meet over a 

 small area at the apical pole, the usual Brechungslinie or cross- 

 furrow of authors. In the later phase (Fig. 10) B and D meet 

 over a wide area. There is now an extensive cross-furrow at 

 the animal pole. A and C meet over a very small area at 

 the vegetative pole. In most forms, e.g., Clepsine, Nereis, 

 Crepidula, if either cross-furrow be the larger, it is that at the 

 vegetative pole. The difference finds its explanation in the 

 fact, that the greater mass of the first four blastomeres is 

 ectodermal in Unio and entodermal in the above mentioned 

 forms. The special character of the cross-furrow is hence due 

 to the accumulation of the specific plasms at their own poles 

 of the Q^g. E. B. Wilson comes to practically the same con- 

 clusion when he says that the reduction of the apical cross- 

 furrow as compared with that at the vegetative pole in mollusks 

 and annelids "stands in obvious relation to the different size 

 of the cells produced at the two poles." 



The orientation of the embryo is now a matter of no 

 difficulty. The large posterior cell D maintains its much 

 larger size in relation to the other cells of the embryo. 

 Its more numerous divisions, moreover, give a most charac- 

 teristic and unmistakable appearance to the posterior end of 

 the Qgg. 



Previous authors have failed completely to understand the 

 significance of these first four blastomeres. Rabl held that 

 A, B, and C were ectomeres, and that D formed all of the 

 entoderm and mesoderm as well as some ectoderm. Flemming 

 expressed no opinion on the subject, and Schierholz followed 

 Rabl. As a matter of fact, each of the four cells contains 

 both ectoderm and entoderm. The mesoderm is formed later 

 from D, but in a totally different way from what Rabl thought, 

 as will be shown later. In addition to ectoderm and entoderm 

 A forms a special kind of mesoblast, for which I shall retain 

 the term larval mesoblast, which I used in my preliminary 

 paper. 



