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 esich 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, 

 (maiosis) 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 diagrammatically 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 



