122 INTRODUCTION TO CYTOLOGY 



(or often haploid) . The set is evidently a definitely differentiated group 

 of elements all of which are usually necessary for normal development. 

 In the male gamete there is a similar set of chromosomes. After the 

 union of the gametes, therefore, the fusion nucleus of the zygote contains 

 two sets of chromosomes. Since every kind of chromosome is present in 

 duplicate, the nucleus is said to be diploid. All of the nuclei of the higher 

 plant or animal body which develops from the zygote are derived by a 

 series of equational mitotic divisions from the zygote nucleus; hence they 

 are all diploid. 



In the case of animals, gametes are formed when the individual 

 becomes sexually mature; in this process the diploid zygotic number of 

 chromosomes is reduced to the monoploid gametic number. This is done 

 in such a manner that each gamete contains one complete set of elements, 

 although this set may include elements from both of the original sets 

 (Chapter XVI). 



In vascular plants, bryophytes, and some thallophytes the situation is 

 somewhat more complicated. The zygote and the sporophyte body into 

 which it develops are typically diploid. A reduction to the monoploid 

 number occurs when reproductive cells are formed, but in such plants 

 these are spores rather than gametes. Each spore develops into a body 

 of another type, a gametophyte with monoploid nuclei. Eventually the 

 gametophyte produces gametes without further change in chromosome 

 number, and these unite to form new zygotes. Hence such a plant life 

 cycle differs from that of a higher animal in having two kinds of repro- 

 ductive cells (spores in addition to gametes) and a monoploid body 

 developing between sporogenesis and gametogenesis. It shows an 

 "alternation of generations" which is usually, though not always, asso- 

 ciated with an alternation of monoploidy and diploidy in the nuclei. ^^ 

 Chromosome morphology in plants has been studied most in root tips, 

 where two chromosomes of each kind are present, but it is often advan- 

 tageous to use gametophyte tissue or spores because a monoploid group 

 is easier to interpret, other things being equal. 



Since the description of the chromosomes of Brachystola by Sutton in 

 1902, morphological differences in the members of the complement have 

 been reported for a considerable number of animals belonging to various 



^1 The two nuclear phases, irrespective of their relation to the "generations," 

 have been called gamophase and zygophase (Winkler). It is generally supposed that 

 in the course of evolution the monoploid condition was first developed through a 

 differentiation of the multiplying nuclear elements, the diploid condition arising as a 

 result of fusion and necessitating a reducing process. The diploid generation is 

 thought to have developed as the latter process became delayed in the cycle and to 

 have hastened the evolution of diverse types by affording larger opportunity for the 

 formation of new combinations through meiosis and syngamy (Svedelius, 1921). 

 The diploid condition is also advantageous in affording a measure of protection 

 against the detrimental effects of unfavorable recessive mutations. 



