THE GAMETES IN PLANTS AND ANIMALS 323 



It was shown in an earlier chapter (pp. 245-47) that dcdiffcr- 

 entiation undoubtedly occurs very commonly in plants, especially 

 in connection with adventitious and experimental reproduction. 

 The new plants thus formed from cells previously differentiated as 

 parts of other plants possess the capacity to form gametes. In 

 other words, gametes may very often arise from cells which form 

 differentiated parts of the plant body, and there is no evidence of 

 the continuous existence of any germ plasm in the theoretical sense 

 in such cells. 



To sum up, we find in the plants no indication of continued or 

 early segregation of germ plasm from somatic plasm. In most 

 cases the gametes are not separated from somatic cells until the final 

 stages of their developmental history, and on the other hand 

 differentiated cells, in many cases every cell of the plant, may 

 undergo dedifferentiation and redifferentiation into new indi- 

 viduals capable of producing gametes. Either all the cells of the 

 plant contain germ plasm or there is no continuity of germ plasm 

 in the plant. The facts point to the second of these alternatives. 

 The gametes arise in the course of development like other specialized 

 parts, and like these also possess a definite history of differentiation. 



THE ORIGIN OF THE GAMETES IN ANIMALS 



In many of the unicellular animals, as in the unicellular plants, 

 the cell which constitutes the organism becomes the gamete. In 

 others the gametes are different in form from the vegetative stages, 



the process of reduction occurring in the formation of the spores which give rise to the 

 gametophyte. But in various mosses and ferns apospory may occur, i.e., the gameto- 

 phyte may arise from other cells of the sporophyte without the occurrence of chromo- 

 some reduction, in which case the cells of the gametophyte, including the egg, possess 

 the full or diploid number of chromosomes. Where the gametophyte jiossesscs the 

 haploid number of chromosomes, apogamy gives rise to a sporophyte with the haploid 

 number, half the number characteristic of sporophytes, but when the gameto- 

 phyte cells are diploid, the sporophyte which arises apogamously or parthenogeni- 

 cally possesses the full number. Various other combinations of apospory, apogamy, 

 parthenogenesis, and fertilization have been recorded. In certain mosses, for example, 

 the aposporous formation of diploid gametes, followed by fertilization and the develop- 

 ment of a tetraploid sporophyte, has been observed (Marchal, '07, '00, '11, '12). The 

 number of chromosomes is evidentlj' not connected in any essential way either with 

 the differentiation of sporophyte and gametophyte or with the formation of the 

 gametes, since any of these stages may possess either the diploid or haploid number. 



