No. 584] 



BILA TERAL1T Y IN VERTEBRATES 



509 



by Balfour, Riickert, Gerbe and Sobotta all show that in 

 these forms such an area is present. In the Reptilia, 

 Vay's studies on Tropirfonoius show that an area of small 

 cells represents the posterior end of the embryo, v. 

 Koelliker first called attention to such an area in the 

 blastodisc of the chick and suggested that it determines 

 the position of the posterior end of the embryo. The 

 later investigations of Duval and Kionka leave no doubt 

 as to the frequent and probably constant appearance of 

 this area in the locality which later becomes the posterior 

 end of the embryo. 



In 1904 the writer made a study of the egg of Necturus, 

 which from its size is especially favorable for surface 

 study. This work was undertaken with a view of ascer- 

 taining how early this secondary area could be located. 

 It was found that as early as the fourth or fifth cleavage, 

 the cells on one side began to divide more rapidly than 

 any others, excepting those of the primary area. It was 

 possible to predict in this form the median plane of the 

 forthcoming embryo at an extremely early stage of 

 cleavage. 



The following year de Bussy from his studies on the 

 Japanese Cryptobranchus emphasized the fact that he 

 could find no secondary area of accelerated cell division 

 such as had been described by the present writer. Yet 

 Smith working on the American Cryptobranchus says 

 that he finds "an accelerated cell division about a radius 

 ©f the blastodisc which gives a condition of bilateral sym- 

 metry." 



The writer felt that it was scarcely necessary to follow 

 the subject further and should not have rehearsed the 

 findings had it not been that certain material came into 

 his hands last year which bears directly upon this sub- 

 ject. This material consists of unpublished descriptions 

 and drawings made by the late Professor C. O. A\ hitman 

 in June, 1894. These were turned over to me by the de- 

 partment of zoology of the University of Chicago. Pro- 

 fessor Whitman's notes run as follows: 



