462 Prof. J. B. Farmer. On the [May 13, 



consideration of such an example as this serves to illustrate the truth of the 

 general statement that the chromosomes, like other organs, are subject to 

 change, both as to size and number. Such a difference is not uncommon 

 between closely allied species ; and in several of the ferns to which I have 

 referred, the number is not even constant in the same individual. It would 

 seem, then, that the bodies in question form rather a frail support for the 

 heavy weight of speculation that has been piled upon them. 



Much of the importance which has been attached to the chromosomes in 

 connection with organisation is connected with the fact that in the plants 

 the process of meiosis is not directly followed, as it is in animals, by the 

 differentiation of sexual cells. In most plants the meiotic phase forms the 

 starting point of a definite stage in the life-history of the organism, and all 

 the cells are distinguished by the possession of the half, or the post-meiotic, 

 number of chromosomes. Thus in the fern, the plant as we know it, 

 possesses the double set. Meiosis is associated with the formation of the 

 spores, and when these germinate they give rise to the sexual plant, the 

 prothallium. All the cell-nuclei of this structure continue to show the 

 reduced or post-meiotic number of chromosomes as the consequence of meiosis. 

 It seemed natural, then, to regard this change of nuclear structure as 

 connected in some causal way with that alternation of a sexual and a sexless 

 generation which is so striking a feature in the vegetable kingdom, especially 

 in the higher classes, of which the ferns may be taken as examples. 



The investigations on apogamy, and especially on apospory, have shown this 

 view to be untenable. Indeed, it has been known for some years that eggs of 

 species of Echinoderms when stimulated to develop parthenogenetically 

 proceed to segment on perfectly normal lines in so far as they can develop at 

 all. Their nuclei retain the post-meiotic number of chromosomes throughout 

 the short life of the early larval stages, that is up to the death of the 

 embryonic animals. 



We have been able to show, in the case of ferns, that in apospory there 

 may be a direct transition from the fern plant to the prothallium, accompanied 

 by the total suppression of the meiotic phase which normally marks the 

 passage from the one generation to the other. The nuclear character of such 

 an aposporous prothallium exactly resembles that of the fern plant from 

 which it sprang, instead of only having half the chromosomes. The chief 

 difference of importance which the prothallium itself exhibits, as contrasted 

 with an ordinary one, lies in its inability to produce fertilised eggs. The 

 sexual organs may be fully formed, and the sperms are frequently found to 

 be vigorously motile. Fertilisation, however, never occurs, and the fern 

 either arises directly from the unfertilised egg, or from the tissues of the 



