NUCLEI OF ANIMAL AND VEGETABLE CELLS. 147 



centre into a substance which originates a partition between 

 the coming segments, and thus corresponds to Strasburger's 

 Zellplatte. 



With the preceding resumes before us, it is not difficult to 

 frame a general account of the earliest stages of cleavage in 

 which all the observers may agree. 



As the ovum approaches maturity, shortly before or about 

 the period of fenilisation, certain changes of regressive 

 character overtake the germinal vesicle, which result in its 

 total or partial destruction. 



Under the influence of the fertilising act the ovum again 

 becomes nucleated, the nucleus arising in a fusion of pro- 

 nuclear bodies, one of which may possibly be derived 

 from the fertilising element. The nucleus at once begins a 

 cycle of changes about the facts of which the unanimity of 

 the various observers leaves us no doubt. It first looses its 

 nucleji and elongates into a spindle, which becomes indistinct 

 and irregular, due probably to the collection about it of a 

 clear or finely granular substance. In the plane of its 

 equator a granular disc or zone of matter more receptive of 

 coloration by staining fluids appears and splits, after a short 

 time, into two parts, each of which begins a movement 

 towards that pole of the spindle which is nearer to it. At 

 the same time, or a little while before, the granular cell-body 

 exhibits in its substance a distinct radiation as of tAvo stellate 

 figures which centre about the tips of the nucleus. 



The two disc- segments gain at length the extremities of 

 the spindle, and a striation of meridian lines is seen to extend 

 between them. Again, in the equatorial plane, a differentiation 

 of structure occurs, and a second granular plate is formed 

 The disc-segments are the nuclei of the new cleavage-spheres, 

 and the second equatorial plate marks out, and assists in, 

 division of the cell-mass, which has meanwhile been pro- 

 gressing inwards from the equator. 



With regard to the germinal vesicle, both Auerbach 

 and Strasburger agree with the numerous observers 

 who describe it as disappearing entirely either before 

 or during fertilisation ; but neither gives in detail an 

 account of its destruction. Hertwig and van Beneden, on 

 the other hand, have entered into the question fully, and 

 have arrived at very difiFerent conclusions. Their descrip- 

 tions of the germinal vesicle in the unripe ova of Toxopneustes 

 lividus and the rabbit agree in all particulars — an agreement 

 which is noteworthy in regard to the granulous reticular 

 threads of protoplasm described as stretching across the vesicle. 



