CHAP, i The Nature of Alternation of Generations 61 



a Fern sporophyte appeared was the birthplace of the 

 vastly developed series of the Phanerogams. The Thallo- 

 phytes hitherto known do not give the least clue to the 

 discovery of that spot.' 



Another factor was brought into the discussion in 1893, 

 which, when followed up, pointed to structural difference 

 between the gametophyte and the sporophyte which was 

 not traceable lower than the Hepaticae. This was the 

 discovery by Overton that the nuclei of the cells of the 

 prothallus of Ceratozamia have only half the number of 

 chromosomes which are possessed by those of the sporophyte. 

 The subject was discussed by Strasburger at the meeting 

 of the British Association for the Advancement of Science 

 at Oxford in 1894, when, after dealing with the cells of 

 gametophytes and sporophytes generally, he gave it as his 

 opinion that the facts then known justified the belief that 

 there is a definite difference between the number of chro- 

 mosomes of the two generations in archegoniate plants, the 

 sexual generation exhibiting half the number of the asexual 

 phase. Strasburger further showed that in most cases 

 the reduction or halving of the larger number takes place 

 in the spore mother-cells. He stated that, in his opinion, 

 this difference is fundamental and phylogenetic, and that 

 the reduction-division implies a return to the original 

 generation, from which, after it has attained sexual differen- 

 tiation, offspring is developed having a double number 

 of chromosomes. Strasburger restated the position in 

 1897, his views closely resembling Bower's, though led up 

 to by a study of nuclear detail. 



The very strong position which was thus taken by the 

 upholders of the antithetic theory was not, however, allowed 

 to remain unchallenged, though but little impression was 

 made upon it before the close of the century. It was 

 attacked with some vigour and no little ingenuity by Scott 

 in 1896. He restated Pringsheim's views, and showed that 



