Ill DIFFERENTIATION 81 



is more serious, possibly because some other material is driven 

 from the animal pole into the interior of the egg, there is no 

 head at all. 



Hertwig and Wetzel have studied the effect of centrifugal 

 force upon the unfertilized egg. 



Similar experiments have been performed on other ova. 

 We turn first to those, carried out by Lyon, Morgan, and 

 Spooner upon the eggs of the Sea-urchin Arbaeia, in which 

 there is a diffuse red pigment. 



If the ripe but unfertilized ovum be strongly centrifuged 

 (/ = 6400 g) four strata appear. The pigment passes to the 

 centrifugal pole, next to this is a grey granular layer, 

 blackened by osmic acid, then a fluid hyaline layer in which 

 lies the nucleus, while the centripetal pole is occupied by 

 a cap of opaque white material. The new axis of stratifica- 

 tion which is thus produced by the operation may make 

 any angle with the original axis as determined by the 

 micropyle. 



When removed from the centrifuge the strata begin to 

 remingle, but the first and fourth return to their original 

 positions very slowly if at all. The second and third layers 

 on the other hand intermingle with one another rapidly, and 

 it is apparently necessary that they should do so before 

 segmentation and development can occur; for if the egg be 

 broken into two portions between them, then neither portion 

 can be fertilized. 



In segmentation it is the axis of stratification which deter- 

 mines the direction of the furrows, since the first three, which 

 are at right angles to one another as in the normal egg, either 

 include or are at right angles to this axis, or, the axis of 

 stratification coincides with one line of intersection between 

 some two of these three divisions. At the next division 

 the micromeres are formed at that intersection of two 

 furrows which is at the anti-micropylar pole or nearest to 

 it (when the axis of stratification is oblique to the original 

 egg-axis). 



It appears therefore that some invisible polarity of the egg 

 has remained unaffected by the centrifugal force, and this 



16S G 



