CHAPTER IV. SPECIAL PHYSIOLOGY OF REPRODUCTION. 781 



divided into two exactly similar parts. Again, when a plant is 

 propagated by a bulb or a cutting, it is still easy to understand 

 that the resulting plant will resemble, as it usually does in detail, 

 the plant which produced the bulb or from which the cutting was 

 taken. But when a plant, a Mushroom for example, is developed 

 from a single minute reproductive cell, representing but an in- 

 finitesimal proportion of the protoplasm of the parent plant, it is 

 not easy to understand how the parental characters can be trans- 

 mitted to the offspring by such apparently inadequate means. 

 The bulk of the structure by which reproduction is effected, would 

 seem to be an expression of the fact that the bulkier reproductive 

 structure (bulb, tuber, etc.) is but a vegetative structure imper- 

 fectly differentiated for the purpose of reproduction, but which, 

 at the same time, ensures a close individual resemblance between 

 parent and offspring: whilst a single minute reproductive cell, 

 on the other hand, such as the gonidium of a Mushroom, though a 

 less efficient instrument of heredity, is much more highly specialised 

 for the work of reproduction. 



If it be asked in what does this specialisation consist, no satis- 

 factory reply can be given at present ; as already pointed out (see 

 p. 766) there is no method by which reproductive capacity can be 

 analysed or determined. It is, however, generally agreed that 

 the transmission of hereditary characters is intimately associated 

 with the nucleus of the reproductive cell, and especially with the 

 fibrillar network (see p. 96) of the nucleus. But the fact that 

 this is so still remains unexplained, as also the fact that repro- 

 ductive cells differ so widely in their properties from vegetative 

 cells, although many attempts at an explanation have been made, 

 such as the theory of Pangenesis and that of Germ-plasm. 



Whilst the ultimate facts of reproduction thus remain unex- 

 plained, the proximate facts of sexual reproduction can be ren- 

 dered intelligible. It has been already pointed out (p. 770) that a 

 gamete is incapable by itself of giving rise to a new individual; 

 but that by the fusion of two gametes of opposite sexes a spore is 

 formed, possessing twice as many chromosomes in its nucleus as 

 did the gametes, from which a new individual may be developed. 

 The nuclei of the new individual (in view of the strict equality in 

 the process of mitotic division, p. 118) must all contain chromosomes 

 derived from both the gametes which had fused in the sexual 

 process ; and if, as seems probable, the transmission of hereditary 

 characters is associated with the nuclear chromosomes, the char- 



