Ch. VI, 4] NATURE OF FERTILIZATION 



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substantially like that which produced it. Thus the nucleus 

 of the fertilized egg cell contains chromosomes in duplicate, i.e. 

 two complete sets, each capable of reproducing an individual. 

 The subject becomes clearer as we consider the events 

 which follow fertilization. The fertilized egg cell at once pro- 

 ceeds to divide. First the chromatin threads collect them- 

 selves into definite chromosomes, equal in number to those 

 contributed by the two nuclei, and indeed, there is little 

 doubt, individually identical therewith. Then each chromo- 

 some splits lengthwise very regularly into two (Fig. 193), 

 whereupon, by action of a spindle-shaped mechanism, 



Fig. 195. — Diagram to illustrate the performance of the chromosomes 

 in the division of the fertilized egg cell. 



On the left the same egg cell shown in Fig. 194, its chromosomes being 

 derived from the two parents ; on the right the two cells resulting from 

 its division. 



one of the half chromosomes in every case is drawn to one 

 end of the cell, and the other half to the other, thus dividing 

 every one of the original chromosomes equally between the 

 two ends of the cell. Then a wall forms across the cell be- 

 tween them, and the two sets merge back each to its chroma- 

 tin tangle. Thereafter these two half-sized cells absorb 

 nourishment and gradually grow to the full original size, the 

 chromatin included. Presently these new cells themselves 

 divide, and the chromosomes which differentiate from the 

 chromatin tangles seem identical with those which went 

 into them, and therefore are the grown-up halves of the 



