88 DEVELOPMENT OF THE FROG'S EGG [Cn. VIII 



abnormally turned, so that the predetermined material takes a 

 definite position, and the blastopore always appears in its proper 

 hemisphere. A rearrangement, Pfliiger believed, does not take 

 place, because the egg, if set free, even after it has been turned 

 for two hours, will tend to rotate into its normal position. Such 

 an egg, set free in its membrane, places the primary axis ver- 

 tical, and this rotation ivill take place even after the first and 

 second furrows have appeared ; and this would not be the case 

 had there been a rearrangement of the contents. 



Pfliiger noticed, however, in eggs that had been turned into 

 abnormal positions, that the upper, white hemisphere is often 

 darker in the later stages than it was at first, and conversely, 

 the black hemisphere may appear lighter owing to the loss of a 

 part of its pigment. This is brought about, Pfltiger believed, 

 by a streaming of the pigment-granules of the egg, and is not 

 a result of the rotation of the contents as a whole. 



The position of the dorsal lip of the blastopore is determined, 

 then, in part by the position of the primary axis, and in part 

 by the tertiary axis, since the blastopore is always in the lower 

 hemisphere, however the egg be turned. " The primary axis 

 determines the meridian, and the tertiary axis the parallel in 

 which the dorsal lip of the blastopore shall appear." 



Since these statements are true for all possible positions of 

 the primary axis, it follows that all primary meridians are of 

 equal value. If we think of an egg with inclined primary axis 

 and imagine this egg rotated around such an axis, then all the 

 primary meridians of the egg will in turn come uppermost. 

 Whichever one is brought to rest in the vertical plane, that one 

 will symmetrically halve the opening of the blastopore when 

 the latter develops, and on that one the embryo will lie with its 

 head turned upwards. It is this vertical meridian that coin- 

 cides with the direction of the force of gravity. In this 

 meridian, every part is not of equal value, because the blastopore 

 appears only in a certain region, and the position of the embryo 

 is thus fixed. The appearance of the blastopore on the vertical 

 meridian below the equator marks the crystallization-point of 

 the whole organization. In other words, the egg-substance has 

 at this time one meridian polarized. Pfliiger says : " I think of 

 each half of the egg after this as polarized, for both halves are 



