DEVELOPMENT OF THE HUMAN BEING 
active state occur the extraordinary processes of the divi- 
sion of the nucleus and of the cell (Fig. 2). Within the 
nucleus at this time the microscope reveals the chromo- 
somes as a certain number of aggregations of granular 
material. These take various forms, such as loops or 
chains or single lumps. The number BF chromosomes in a 
cell is characteristic and is constant for each species, but 
within each species it may differ between the two sexes. 
In man the count is difficult, but the number is usually 
regarded as forty-eight. 
Preparatory to cell division, a pair of centers migrate 
apart to opposite poles of the nucleus (Fig. 2). The chro- 
mosomes range themselves as if upon a central plane with 
respect to these centers, while from each center rays or 
fibers go out and fasten upon the chromosomes, thus giving 
a spindle-shaped appearance to these radiating threads. 
Thus the whole structure at this time resembles a double 
cone, with the company of chromosomes at the junction of 
its fees. Meanwhile the membrane which inclosed the 
nucleus has dissolved, so that the fluid of the nucleus 
merges into the cytoplasm of the remainder of the cell. 
And now all of the chromosomes are dragged into halves, 
as though drawn toward the two centers by opposing pulls 
of the connected fibers. These half-chromosomes come 
together near each of the centers and about them new in- 
closing membranes are formed. The outer membrane of 
the cell itself then shows a furrow which deepens into a 
middle septum, and then the cell divides into two cells, 
each inclosing one of the daughter nuclei. 
Such, in brief, is the general story of all kinds of cells and 
of their multiplication. But in the formation of the special 
male and female germ-cells which unite to produce the 
embryo, each rejects one-half the number of its chromo- 
somes at a certain stage. The act of impregnation com- 
pletes the structure and unites in a single normal cell the 
chromosomes of two cells, which were distinctively male 
and female before impregnation. In every division of 
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