Lecture XIV. in 



a gamete without fertilisation is technically known as partheno- 

 genesis. 



In some species of Fucus the antheridia and the oogonia 

 develop in the same conceptacle, in others they are formed in the 

 conceptacles of different plants. 



These unisexual (dioecious) individuals lend themselves to 

 interesting experiments. Supposing the ova of one of these 

 dioecious species be collected in a watch-glass in salt water and 

 into the same glass the sperms of a second dioecious species are 

 introduced, fertilisation will take place and hybrid oosperms 

 result. When the plants grow to maturity it is found that they 

 are intermediate in their characteristics between the two parents. 

 If now the sperms of the first plant are added to the ova of the 

 second, similar hybrids will also be produced and these hybrids are 

 identical with those produced in the first experiment. One might 

 be inclined to think that such a result was only to be anticipated 

 and pass on without further reflection. But this simple observation 

 shows that the characters of the parents are transmitted in the 

 gametes, notwithstanding their minute size ; and further, that these 

 characters are equally represented in the excessively minute sperms 

 and in the larger ova, and that the influence of the sperm on the 

 development of the embryo is as potent as that of the ovum. 

 Hence it is probable that the amount of hereditary material 

 contributed by the sperm is equal to that contributed by the ovum. 

 Owing to the unequal amount of cytoplasm in the two cells it 

 seems highly improbable that the cytoplasm is the hereditary 

 substance and controls the development of the offspring. While 

 the bulk of cytoplasm in the two gametes is very different it may 

 be observed that the bulk of granular material in the nuclei 

 (chromatin) of the two gametes is at least approximately equal. 

 This observation suggested to investigators that the chromatin of 

 the nucleus is the hereditary substance or germ-plasm, and the view 

 is supported by many other considerations, some of which will be 

 spoken of later. 



If the chromatin of the nucleus of the gametes is the germ-plasm 

 then we may with equal probability assume that the chromatin of 

 the nucleus of a ispore is its germ-plasm. Wherein then may 

 we expect reproduction by spores to differ from that by gametes ? 

 Evidently the spore contains the germ-plasm of one individual 

 only, while the oosperm, or zygote, contains that of two. Offspring 

 developing from an oosperm will be the mean of two individuals 

 so that idiosyncrasies will tend to be averaged out, while the 

 individual peculiarities of the single parent may be looked for in 



