GROWTH IN LENGTH OP THE FROG EMBRYO. 233 



In the frog the dorsal lip cannot of itself grow outwards and 

 so produce an oval embryo until the rest of the blastoporic 

 lips are formed. Unless it remains inactive it must follow the 

 contour of the ovum. That it does not remain inactive I have 

 convinced myself^ and therefore I agree with the above- 

 mentioned authors that a portion of the white area passes out 

 of view of the observer by becoming hidden by the advancing 

 dorsal lip of the blastopore. 



Experiments in marking Parts of the Ovum. 



General Remarks. — I find — 



(i) That it is impossible to fix the egg in any one position 

 so as to prevent with certainty the rotation of the ovum 

 within the vitelline membrane without injuring or distorting 

 the ovum. 



(ii) That, accordingly, any fragment which exudes from the 

 ovum through the aperture made in the vitelline membrane 

 when pricking the ovum in order to mark one spot is useless 

 as a landmark. 



(iii) A scar upon the ovum itself, fixed to the ovum and 

 within the vitelline membrane, is the only mark which can be 

 relied upon for drawing conclusions as to the relative rate of 

 growth, and a change of position at difi'erent points upon the 

 surface of the ovum. 



(iv) A severe injury by pricking naturally produces much 

 abnormality of development; whereas a very slight injury, 

 although admirable for a short observation, is apt to recover 

 and so get lost and obliterated after many hours. 



Some of my own experiments I will now briefly describe. 

 Outline figures are given upon PI. 23. 



Figs, la — Id show the results of an experiment. Here the 

 puncture was very small, and made midway between the two 

 horns of the developing blastoporic lips (fig. la). Three 

 hours and a half later the blastoporic lips were completely 

 marked. By this time (fig. lb) the mark was distinctly closer 

 to the dorsal lips of the blastopore. Three hours later the 

 mark had approached the dorsal lip still nearer, but the ventral 



