FUNDAMENTAL PRINCIPLES 187 
membrane then disappears, and, by a complicated mechan- 
ism, not entirely understood, the two halves of the chro- 
mosomes are separated and carried apart to opposite sides 
of the cell. After this division of the nucleus, a new cell- 
wall forms, dividing the entire cell into halves ; new nuclear 
membranes develop, and the chromosomes in each daughter- 
nucleus become gradually retransformed into a resting 
nucleus, like the one with which we started. 
In reduction (Fig. 137) a new resting nucleus is not 
organized after the first nuclear division, but this divi- 
sion is followed at once by a second, or reducing division, 
by which the number of chromosomes in each nucleus is 
reduced by one-half. This is the process of tetrad-division, 
by which spores are formed from the spore-mother-cells. 
The reduced number of chromosomes persists throughout 
the gametophyte-phase, including the formation of both 
egg and sperm. When the latter unite the nucleus of the 
zygote will, of course, possess the doubled number of 
chromosomes, which then persists throughout the body of 
the sporophyte (mature zygote), until the stage of spore- 
formation is again reached. These facts are shown dia- 
grammatically in Fig. 138. 
169. Inheritance. It is, of course, common knowledge 
that men do not gather grapes of thorns, nor figs of 
thistles. A given species of fern always reproduces the 
same species, and this is true of all plants. It requires 
only a brief reflection to realize that this must be so, for 
the beginning of every living thing is always merely a 
piece of an antecedent organism, the parent. The off- 
spring would, therefore, naturally partake of the nature of 
its parent it is a piece of it was originally a part of 
it. Resemblance between ancestor and descendant is 
