DIV. i MORPHOLOGY 203 



of chromatophores is maintained, disappear. In Angiosperms, so far 

 as our present knowledge goes, only a male nucleus, without cytoplasm 

 or chromatophores, enters the oosphere. From this it has been 

 concluded THAT THE ESSENTIAL ELEMENT IN FERTILISATION is THE 



PASSAGE OF THE MALE NUCLEUS INTO THE EGG-CELL. 



In the typical process of nuclear divisioji it has been seen that 

 the nuclei of an individual possess a constant number of chromosomes 

 characteristic of the species. The male gamete thus contributes as 

 many chromosomes as the female gamete. These chromosomes do 

 not fuse in the conjugation of the sexual nuclei, so that the nucleus 

 of the zygote has double the number of chromosomes possessed by 

 the sexual cells { l02 ). It is DIPLOID and contrasts with the HAPLOID 

 nuclei of the gametes. 



The nuclei resulting from the further division of the nucleus of 

 the zygote are as a rule diploid ; in each there are as many chromo- 

 somes derived from the male as from the female nucleus. When 

 the chromosomes of the haploid cells are characterised by differences 

 in size which are apparent at each nuclear division, the diploid 

 nuclei show pairs of chromosomes of each size. These chromosomes 

 of equal length, the one derived from the male and the other from 

 the female parent, as a rule lie in pairs in the nuclear plate (Fig. 1 4). 



Since the nuclei of the sexual cells of all the individuals of a 

 race are always haploid, while the conjugation nucleus and as a rule 

 the products of its division are diploid, there must be a change from 

 the diploid to the haploid condition at some point in the developmental 

 history of the individual. Were this not so, the number of chromo- 

 somes would double with each generation. The change is effected 

 at the REDUCTION DIVISION ( 103 ), which is a peculiar nuclear division 

 in which there is a separation to form the daughter nuclei of entire 

 chromosomes, and not half-chromosomes resulting from longitudinal 

 splitting. This occurs at a definite point in the development, which, 

 however, differs in different organisms. Thus a regular alternation 

 of the haploid and diploid phases of the nucleus is characteristic of 

 the ontogenetic development of sexual organisms. 



Frequently, but not always, the alternation of nuclear phase 

 is connected with the alternation of generations, as in many Algae, 

 Fungi, the Bryophyta, Pteridophyta, and Spermatophyta. The 

 sporophyte arising from the fertilised egg is diploid, and the reduc- 

 tion division precedes spore formation. As a result the spores, the 

 gametophytes developed from them, and the sexual cells are haploid. 



In many Algae, however, the first division of the nucleus of the zygote is the 

 reduction division, so that all the cells of the organism, including the sexual cells, 

 with the exception of the fertilised egg, are haploid. In others, such as Fucus, 

 the reduction takes place at the formation of the sexual cells, so that the opposite 

 case is presented of all the cells with the exception of the gametes being diploid. 



There are certain remarkable cases in which the one generation develops from 



