THE ORIENTATION OF THE FROG's EGG. 393 



Review of Literature. 



There are certain statements made in the papers of Pfliiger, 

 Roux, Schultze, and E/obinson and Asshetonthat bear directly 

 on the results given above. Pfliiger records that in one set of 

 eggs the blastopore first appeared at 6 a.m. At 11 a.m. the 

 blastopore was broader, with the corners turned down. The 

 blastopore had left the equator of the egg and approached to 

 the lower pole. At 12.30 p.m. the blastopore was semicircular, 

 and had pushed further towards the lower pole. At 1 p.m. it 

 was circular, and now it lay at the opposite point of the white 

 hemisphere from which it had started. 



An examination of the relation of the pigment shows that 

 the egg as a whole has had no part in this overgrowth of the 

 lower pole, i. e. no rotation of the whole egg has taken place. 



At 2.15 p.m. the yolk-plug was smaller, and the blastopore 

 has continued to move in the same direction. At 4.15 the 

 blastopore is narrower still, and its diameter equals about one 

 eighth the diameter of the egg ; it has moved even further, 

 and is in the region of the equator of the egg, but at the 

 opposite point of the equator from which it started in the 

 morning. 



These observations point conclusively to the view that '^ the 

 opening of Rusconi passes from a point on the equator lying 

 in the meridian of the egg over to the opposite point of the 

 equator through the lower white hemisphere, and the egg-axis 

 during the period has not changed its position." 



The arc travelled is not quite 180°, but is certainly more 

 than a right angle, — variable, however, in different eggs. The 

 overgrowth is due to a process of invagination. 



From 4.15 p.m. till 7.45 p.m. the egg as a whole rotates in 

 the opposite direction along the same meridian. Due to this 

 true rotation more than one half of the (new) upper hemisphere 

 is covered by those cells that overgrew the blastopore, and 

 which therefore have a lighter colour than the cells of the 

 primary upper hemisphere. From this clearer portion in front 



