THE ORIENTATION OF THE FROG's EGG. 373 



The Orientation of the Frog's Egg. 

 By 



T. H. Morg:aii, Ph.D., 



Associate Professor of Biology, Bryn Mawr College ; 



AND 



Ume Tsuda, 



Teacher in the Peeress' School, Tokio, Japan. 



With Plates 24 and 25. 



I. 



The classical experiments of Pfliiger on the segmenting 

 frog's egg, and the important conclusions drawn by Roux 

 from a study of the same egg, have made it very desirable to 

 have an accurate knowledge of the relation existing between 

 the early segmenting egg and the position of the embryo with 

 respect to the egg. 



The interpretation of certain embryos in which the blasto- 

 pore has failed to close, recorded by Roux and Hertwig, will 

 likewise depend on the normal position of the embryo on the 

 egg. Pfliiger, Roux, and Hertwig have come to the conclusion 

 that the embryo forms over that portion of the uusegmented 

 egg which is normally directed downwards, i. e. over the white 

 hemisphere. Schultze supports the old view, that the embryo 

 lies on the upper or black hemisphere. 



Pfliiger based his conclusion on the evidence obtained by 

 actually following the dorsal lip of the blastopore in its migra- 

 tion over the white hemisphere. Roux based his conclusion on 

 evidence obtained by destroying definite portions, both of the 

 segmented and unsegmented eggs. 



