THE ORIENTATION OF THE FROG's EGG. 391 



growth (20 mm.) is less than the diameter of the circle (27 

 mm.). Comparing this with the length of the medullary folds 

 when they first appear, the area overgrown is found to be some- 

 what less in length. If we deduct from the length of the embryo 

 the thickness of the medullary folds at their anterior border, 

 we find that the length of the two regions corresponds almost 

 exactly. In other words, the connection around the anterior 

 end of the medullary folds lies just in front of the point where 

 the blastopore first formed, and the area overgrown by the 

 dorsal lip equals the length of the medullary folds between the 

 anterior connection and the blastopore. 



A few corrections should be made ; the measurements just 

 given apply only to the flat surface, while the embryo lies over 

 a spherical surface. As the measurements of the overgrowth 

 and the measurements of the embryo are both projections into 

 the same plane, no gross error will come into the calculation. 

 The rate of overgrowth is not quite the same in all the ob- 

 servations, but approximately so. Even the extent of over- 

 growth is variable, and we have seen that the length of the 

 embryo formed is also variable. 



The first overgrowth of the dorsal lip of the blastopore is 

 more rapid than the later growth ; that is, the approach to the 

 point of injury is faster at first. After the blastopore has 

 completed its circular outline the process of overgrowth (or 

 withdrawal) of the yolk-plug is much slower. 



I have assumed the point of injury to be the fixed point, 

 and the approach of the blastopore to be due to the movement 

 of the latter. We might have assumed that the overgrowth 

 was due to a forward movement of the whole of the white area 

 passing beneath the blastoporic lip. The end result would be 

 the same in either case, the process diflFerent. It is not an 

 easy question to decide, but to any one following the process 

 in the living egg it will be clear that the change is due to the 

 movement of the blastopore lips, and not to the white area. 

 The condition of the cells in the white area points to a relative 

 stability and inertness, while the reverse is true for the dorsal 

 lip of the blastopore. The method of invagination of the 



