1907.] Structural Constituents of the Nucleus, etc. 463 



prothallium. In such a case, then, the entire life-history is passed through 

 without any reduction of the chromosomes ; the egg, like the other cells of 

 the prothallium, already contains the full complement, which normally is not 

 supplied until fertilisation. There is, therefore, no reason to be surprised at 

 the omission of fertilisation in these cases. 



It is sometimes possible to induce prothallia which are normal as regards 

 their nuclear contents, i.e., are reduced, to produce embryos apogamously. 

 But in all these instances, excepting one, we have found that the normal 

 sexual fusion, which is here prevented from taking place, is replaced by a 

 fusion of nuclei from adjacent vegetative cells. In this way the normal 

 doubling of the chromosomes is secured. This circumstance has been used as 

 an argument to prove that at any rate the double set is necessary for the 

 production of the sporophyte generation. 



We have encountered one instance, however, in which there seems no room 

 for doubt that the fern plant sprang from a prothallium of post-meiotic 

 structure, without any preparatory nuclear fusion. The plant in question* 

 appeared in a close fernery, and, from what we know of its history, it must 

 have started as a spore. Presumably, therefore, it had passed through 

 meiosis, and this is borne out by the small number of chromosomes present 

 in its nuclei. From the prothallium, fern plants were produced apogamously, 

 and they are all characterised by the possession of the reduced, i.e., the post- 

 meiotic, number of chromosomes. 



It thus becomes evident that, given a complete set of normal chromosomes, 

 whether in single or in duplicate, the entire life-history may be passed 

 through. The peculiar fusions between the nuclei of adjacent prothallial 

 cells, together with the extreme rarity of the formation of the fern plant 

 without the double set, may perhaps be taken to indicate some normal 

 relation as existing between the duplicate number and the sporophyte, but it 

 clearly cannot be of any fundamental importance. 



The fact that both stages of the life-history can be gone through in the 

 absence of that nuclear change which ordinarily corresponds to each one of 

 them is a proof that the organisation of the species, whatever may be the 

 phases through which it passes, can be completely effected either with the 

 single, or with the double, set of chromosomes ; but the meaning attached to 

 sexuality, as importing into the organism the means of producing variation, 

 becomes even more clearly denned than before. 



I desire, in conclusion, to obviate a possible misapprehension which, 

 perhaps, might arise from the way in which I have spoken of the chromo- 

 meres as the agents responsible for the appearance of the characters 

 * Lastrea pseudo-mas cristata apospora, see 'Annals of Botany,' vol. 21, p. 180. 



