SUPPLEMENT 115 



the unreduced number of chromosomes. In other cases certainly, as in Hiera- 

 cium (ROSENBERG, 1906, 1907), normal fertilization takes place as well as 

 parthenogenesis, but it has been shown in this genus that there are two kinds 

 of ova, some with the reduced number and some with the double number of 

 chromosomes, and it is very probable that the former develop only after 

 fertilization, and that the latter are parthenogenetic. 



Since, therefore, it cannot be denied that there is some connexion between 

 parthenogenesis and the number of chromosomes, still it appears to us by no 

 means proved that fertilization removes the inhibition of development by 

 increasing the number of chromosomes. We would draw attention to three 

 groups of phenomena which go to prove that growth is also possible where the 

 reduced number is present. Take first the prothalli of Pteridophyta. In this 

 group of plants, the reduced number of chromosomes is not limited, as it is 

 in Angiosperms, to the ovum and its sister cells, but is characteristic of the 

 whole sexual generation, capable in itself of unlimited growth and of pro- 

 pagating itself vegetatively. Since then one cell of this generation, i. e. the 

 ovum alone, shows inhibited development, this must be due to some special 

 reason. Further, in Angiosperms also, growth of cells with the reduced number 

 of chromosomes is not impossible ; as, for instance, in the pollen-tube, Which 

 it is true does not show unlimited powers Li this respect. 



Secondly, we may note the cases of artificially induced parthenogenesis, 

 i.e. parthenogenetic development of normal ova which possess the reduced 

 number of chromosomes, and which are usually fertilized. In the animal world 

 the experiments of LOEB (summarized 1906) are especially noteworthy, and 

 from these it would appear that ova capable of fertilization may be induced 

 to undergo parthenogenetic development when treated with salt solutions of 

 definite concentration and by withdrawal of water. It is probable that there 

 are plant ova also which may be induced to act in the same way by using 

 appropriate agents. At present none such are known, for STRASBURGER (1907) 

 has shown that NATHANSOHN'S (1900 a) statements as to artificial partheno- 

 genesis in Marsilia are erroneous. We must, therefore, consider lower plants 

 where not infrequently sexual cells are able to develop without fertilization. 

 A case of this kind has been investigated by KLEBS (1893), i.e. in Protosiphon, 

 where development, without fertilization, is induced by high temperature. 



Lastly, we have the phenomenon of so-called ' merogeny ' the develop- 

 ment of portions of ova, more especially those which are without nuclei. Thus 

 WINKLER (1901) observed normal development taking place in fertilized but 

 non-nucleated ova of Cystoseira barbata, and it can scarcely be doubted that 

 they contain only the reduced number of chromosomes brought to them by 

 the spermatozoid. This is quite definitely proved in the case of BOVERI'S 

 experiments with the eggs of sea-urchins, in which the larvae were smaller, 

 but in every respect normal, although the nuclei of these cells contained only 

 half the number of chromosomes present in the cells of normal individuals. 



From all these considerations we may conclude that the initiation of 

 development in fertilization is of the nature of a stimulus, which we may 

 associate with the effect of ether on the unfolding of resting buds. Such 

 developmental stimuli have been the subject of previous consideration, so that 

 we may limit ourselves at present to recording a few examples of stimuli 

 actually provided by pollen-tubes, which have nothing to do with the actual 

 process of fertilization, i.e. the fusion of the two sexual cells. The best known 

 case is that of the Orchidaceae, where indeed the ovule is formed only after 

 the pollen-tube has germinated on the stigma. This stimulus, 



370, 1. 16, after orchids, read According to STRASBURGER (1886) in the case 

 of Fritillaria persica it may also be induced by the pollen-tube. 



H 2 



