THE NATURE OF ALTERNATION, ETC. 337 



apparatus end, which undergoes division without previous 

 fusion with the corresponding nucleus. Within the pro- 

 thallus which thus results the embryo is developed apogam- 

 ously from a cell which occupies a certain position. It 

 is of considerable importance as bearing on the nuclear 

 changes which precede apogamy that the fusion of the polar 

 nuclei (which in other cases is known to result in the 

 doubling of the number of chromosomes in the nuclei of the 

 endosperm) does not take place. The exact determination 

 of the nuclear changes in this or similar cases would be of 

 great interest. 



THE DEVELOPMENT OF THE THEORIES OF ANTITHETIC 

 AND HOMOLOGOUS ALTERNATION. 



The nature of the alternation in Thallophytes was dis- 

 cussed by Vines 1 in 1879. His conclusion was that it is 

 impossible to detect any distinct alternation in any but the 

 Coleochcetece and Characecs. In the latter the pro-embryo 

 was regarded as the sporophyte, the main shoot representing 

 an aposporously produced oophyte." 



The views of Naegeli 3 differed from those of both 

 Celakovsky and Pringsheim as to the divisions of the life 

 cycle of the Thallophyta which are to be regarded as 

 generations, and in the comparisons instituted between these 

 and the two generations of Archegoniate plants. A full 

 discussion is impossible, but the views expressed with regard 

 to the alternation in Ulothrix or CEdogonium on the one 

 hand and the Moss or Fern on the other will render the main 

 point clear. The Ulothrix or CEdogonium plants repro- 

 ducing asexually by zoospores are the repetition generations 

 (B 1 — B n ) ; they are followed by one composed of individuals 

 in the main similar to them, but bearing the sexual organs 



1 Vines (2). 



2 Vines (1), (2), (3). In connection with this view the fact that a 

 reduction of chromosomes does not precede the development of the 

 spermatozoids may be mentioned (Debski). Compare also Strasburger, 



(3). P- 413- 



3 Naegeli. 



