394 BERTRAM G. SMITH 



('96, '97) had previously shown that by constricting these eggs 

 in the two-cell stage by means of a noose of fine hair tied around 

 the egg in the plane of the first furrow, it was sometimes possible 

 to obtain two complete embryos of rather more than half size. 

 Spemann showed that this result could be obtained only in those 

 occasional cases where the first cleavage furrow coincides with the 

 median plane of the embryo. 



In the living egg of Necturus, Eycleshj^mer ('04) was able to 

 keep the first cleavage furrow in the lower hemisphere under 

 observation until the blastopore appeared. In the twenty-two 

 eggs studied there was no fixed relation between the median plane 

 of the embryo and the early cleavage furrows. The absence of 

 a constant relation between the first cleavage furrow and the 

 median plane of the gastrula has been demonstrated for the egg 

 of Crypt obranchus by a variety of methods as recorded in the 

 present paper. 



In an extensive series of observations on the living, segment- 

 ing eggs of Ambylstoma, Diemyctylus, Rana, and Bufo, Jordan 

 and Eycleshymer ('94) found that the first and second cleavage 

 furrows undergo extensive torsion. This phenomenon seemed 

 to the authors sufF.cient basis for the conclusion that the early 

 cleavage planes and the embryonic axes have no vital connec- 

 tion, and that the coincidence, where it exists, is of no funda- 

 mental significance. If we assume that material for right and 

 left halves of the body is segregated on opposite sides of either 

 the first or the second cleavage furrow, then as a consequence of 

 the shifting of micromeres we shall later find some of this material 

 crossing the median fine. Eycleshymer's ('04) later study of 

 cleavage in the living egg of Necturus revealed a similar irregu- 

 larity. Extensive shifting of micromeres and torsion of cleavage 

 furrows occurs in the early stages of segmentation of the egg of 

 Cryptobranchus. 



D. The bilateral symmetry of the blastula. While the direction 

 of the early cleavage furrows in the amphibian egg is thus shown 

 to be without causal relation to the median plane of the embryo, 

 bilaterahty is indeed sooner or later made manifest in the cleavage 

 pattern as a consequence of more rapid cell division on one side 



