No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 439 



(Pfliiger), and in Diemyctylus, Jordan found that the first 

 cleavage plane often deviated considerably (45-50°) from the 

 transverse axis. 



In Amblystoma, Jordan and Eyclescheimer found that the 

 first furrow became so irregular that they considered it improb- 

 able that it should ever come to separate, exactly, the right and 

 left halves of the embryo ; and the same conclusion may well 

 be drawn from the cleavage of many other vertebrates. Where, 

 in eggs with bilateral cleavage, the first cleavage plane forms an 

 oblique angle with the median axis, this angle varies in differ- 

 ent eggs, i.e., the first cleavage plane does not stand at a con- 

 stant oblique angle to the median axis. When the first cleavage 

 plane has constant axial relations, it is either median or, more 

 rarely, transverse (Polychoerus, Gardiner, '95). Determinate 

 bilateral cleavage may readily be conceived to occur in which 

 the first cleavage furrow stands at a certain constant oblique 

 angle to the median axis, the second at right angles to the first, 

 and only the third or fourth meridional cleavage furrow coin- 

 ciding with the median plane of the embryo. But the course 

 of bilateral cleavage does not run in this manner. Where, as 

 in the toadfish, the first cleavage plane may form a considerable 

 angle with the median axis, and the cleavages, up to a certain 

 period, are symmetrical in relation to this plane, it is obvious 

 that, unless an extensive shifting of the blastomeres occurs, the 

 cleavage cannot be of the determinate type ; cells of correspond- 

 ing lineage cannot have the same fate in different eggs. In 

 cases like this the form of cleavage can hardly be held to 

 express a bilateral organization of the ^^^ substance and can- 

 not have much morphological significance. Even in eggs devoid 

 of bilateral organization the cleavage might form, according to 

 Sach's law, a more or less definite pattern, whose form would 

 be largely dependent on the amount of yolk in the ^^g, extrin- 

 sic conditions, etc.; or the organization of the &gg may be such 

 as to exert no influence on the early cleavage. But in cases 

 where bilateral cleavage is determinate, where cells of the same 

 lineage always have the same fate in different eggs, the first 

 cleavage plane apparently always coincides with either the 

 median or the transverse axis of the embryo. Where the first 



