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in diameter. It consists of yellowish rather cloudy proto- 

 plasm, sufficiently transparent to permit one to observe 

 the more conspicuous changes which take place in the sub- 

 stance of the living egg. Segmentation is total and equal, 

 of the type which is designated by Metschnikoff as " durch - 

 schneidende Fu re hung ". The cleavage-furrow appears at one 

 side of the egg first and cuts through its substance until 

 it reaches the opposite side, dividing it into two hemis- 

 pheres (Fig, 5), The point at which the furrow starts is 

 that nearest the nucleus, which lies eccentrically in the 

 granular substance of the egg. The first indication of the 

 furrow is a shallow groove which deepens rapidly and at the 

 same time lengthens so as to embrace the egg meridionally. 

 The furrow is finally completed, superficially, a short 

 time before it has entirely separated the egg into two dis- 

 tinct halves. The last point to be cut off corresponds in 

 position almost exactly with the nucleus, but on the oppo- 

 site side of the egg. This first cleavage is completed one 

 hour after fertilization. The two daughter-nuclei now lie 

 near the plane of fission, and at the same distance from 

 the surface as the original nucleus was. The second furrow 

 normally starts at the same point on the surface as the 

 first, and again divides the egg meridionally, in a plane 



