40 ART. 3. S. IKEDA : CONTRIBUTIONS 



cleavage plane and is at right angles to the first. I am inclined 

 to think that the first segmentation cleavage divides the substance 

 of the egg into two parts corresponding to the dorsal, and ventral 

 halves of the future embryonic body, supposing that there is on 

 the whole no interchange of substance during the course of 

 development. 



III. 



EXPLANATION OF THE FACTS OBSERVED IX THE SECOXD 



PART BY CHANGES IX THE EXTERIOR OF THE EGG 



BROUGHT OUT IX SECTIOXS. 



Method : — The Bhacophorus egg presents several peculiar dif- 

 ficulties for sectioning : the closely applied chorion and the 

 innermost envelope of the frothy substance become exceedingly 

 tough, when hardened, so that it is very difficult for paraffin to 

 penetrate into the egg. Moreover, the egg being absolutely with- 

 out pigment, becomes so translucent, when clarified in turpentine 

 or xylol, that orientation becomes impossible. Again, owing to 

 the extreme tenuity of the roof of the segmentation cavity, it is 

 apt to cave in when the egg is transferred from one liquid into 

 another of a different specific gravity. 



After some experiments, the mode of procedure I. adopted 

 was as follows : — The orientation of the egg was secured by first 

 fixing it in any desired position on a thin triangular sheet of 

 frog's liver by a drop of albumen. This was then passed through 

 the ascending grades of alcohol, being kept 13 or 14 hours in 

 absolute alcohol at the end. If a shortening of the time was 

 desired, absolute alcohol was changed two or three times in one 



