362 EYCLESHYMER. [Vol. X. 



wird beim Froschei zugleich auch die kiinftige Medianebene 

 des Individuums bestimmt, und zwar fallen beide zusammen." 

 From the facts recorded by Roux (pp. 14-16) it is evident that 

 this coincidence was by no means constant. 



In the same year Pfliiger {'83) reached the conclusion that 

 the first furrow represented the median line of the embryo. 

 Supported by these eminent investigators, the idea received 

 very wide acceptance. Rauber ('86) was one of the first to 

 question the observations, and showed that in the Frog and 

 Axolotl this was not true, but that the second meridional in- 

 stead represented the median plane of the embryo. By con- 

 sulting Rauber's tables it will be observed, however, that the 

 embryo was formed at almost any angle to the first cleavage. 

 The later experiments of Hertwig on the ^gg of Triton, in 

 which a silk thread was tied through the first furrow, confirmed 

 the observations of Rauber, as do also the observations of 

 Jordan on the Newt. 



The work of Miss Clapp on so favorable an ^gg as that of 

 Batrachus seems to show that in this form, at least, the first 

 plane of cleavage bears no definite relation to the median plane 

 of the future embryo. 



In 1892 Roux modified the position which he held in 1883, 

 and states that " Bei den bilateralen-symmetrischen Tieren 

 entspricht eine der beiden ersten Teilungsebenen der Median- 

 ebene des Embryo resp. des erwachsenen Tieres." 



In case the second furrow coincides with the median plane, 

 it is interpreted as being intrinsically the first. Roux says : 

 " Es entsteht dann die normalerweise zweite, kopf- und schwanz- 

 warts scheidende Furche als erste, und die normale erste, der 

 Medianebene entsprechende Teilungsebene als zweite." 



To my mind a serious objection to all theories postulating 

 the coincidence of any cleavage furrow with the median plane 

 of the embryo in Rana, Bufo, Amblystoma, and Diemyctylus 

 is the fact, emphasized by Jordan and myself ('94), that the 

 furrows undergo a decided torsion. A glance at PI. XVIII, 

 Figs. 13-18, will illustrate the point. This shifting of the 

 cells, although often observed, has hitherto received but little 

 attention. It was witnessed by von Baer in 1834, and described 



