August 18, 1905.] 



SCIENCE. 



209 



proof does not appear. Fig. 1 is the diagram 

 which Professor Lyon fails to reconcile with 

 the statement just made in the first paragraph 

 of this note. Just why he failed to grasp 

 the situation will be evident to any morphol- 

 ogist. He simply failed to distinguish be- 

 tween a gametophytic generation and an indi- 

 vidual gametophytic plant. According to 

 Strasburger^ the spore mother-cell is the first 

 cell of the gametophytic generation. This 

 spore mother-cell usually produces four spores, 

 each of which, with or without germinating, 

 is an individual gametophytic plant. In Fig. 

 1, C and C, this condition is compared with 

 the egg and the three polar bodies. In the 

 same figure, B and B', and A and A' show 

 previous stages in the development. A stage 

 preceding A and A' might have been added to 

 compare the spore mother-eell with the pri- 

 mary oocyte. The diagram is intended to indi- 

 cate not only that the entire gametophytic 

 generation in plants is comparable with the 

 entire gamete-producing generation in ani- 

 mals (this generation beginning with the 

 primary oocyte and ending with the egg and 

 three polar bodies), but also that each of the 

 four megaspores is comparable with an ani- 

 mal egg, the three polar bodies, of course, be- 

 ing regarded as eggs. I am aware that some 

 cytologists, notably investigators of the Gre- 

 goire school, are questioning whether the 

 gametophytic generation does not begin with 

 the spore rather than with the spore mother- 

 cell, since it is not until the spore is reached 

 that the reduced number of univalent chromo- 

 somes is found. This, however, is a minor 

 detail which does not affect essentially the 

 theory presented in Strasburger's paper on 

 periodic reduction or the theory advanced in 

 my paper on alternation of generations. 



The remark ' That zoologists recognize an 

 alternation of generations in ITydrozoa and 

 Scyphozoa is a common statement of their 

 text-books,' coming from a teacher of botany, 

 is rather surprising. We shall take a chari- 

 table view and hope that it was ignorance of 

 zoology rather than of botany that allowed 



^ Strashurger, Ed., ' The Periodic Reduction of 

 Chromosomes in Living Organisms,' Annals of 

 Botany, 8: 281-316. 1894. 



the remark, for the condition in Hydrozoa de- 

 scribed by zoologists as an alternation of gen- 

 erations is not an alternation of generations 

 in the botanical sense, but is only a case of 

 polymorphism, the relation of the medusa 

 form to the parent plant being somewhat like 

 the relation of the leafy moss plant to the 

 protonema. Since my paper appeared, several 

 zoologists have called my attention to this 

 alternation of generations in hydroids, but 

 they have recognized at once that the term, 

 alternation, is used in a totally different sense 

 by botanists and zoologists. 



The general criticism that there is no evi- 

 dence in favor of my theory, would require a 

 long answer. In replying to zoologists who 

 have written to me and in explaining my 

 theory more fully to zoologists whom I have 

 met, the series of diagrams shown in Fig. 2 

 has been useful. The diagrams and explana- 

 tions are given, not as a reply to Professor 

 Lyon, but as a general answer to those who have 

 asked about the progressive reduction of the 

 gametophyte. While the series does not prove 

 that the egg with its polar bodies constitutes 

 a reduced generation comparable with the 

 gametophytic generation in plants, it does 

 indicate how a condition quite strictly compar- 

 able with the animal egg and polar bodies has 

 been reached by the gametophytic generation 

 in plants. 



In Fig. 2 the smaller diagram at the right 

 in each case represents the egg and the three 

 polar bodies. 



In an homosporous fern (Fig. 2, A) each of 

 the four spores derived from the mother-cell 

 may form an independent plant. Four gamete- 

 bearing plants are shown. Professor Lyon's 

 confusion probably arose from his failure to 

 recognize that the gametophytic generation 

 could include anything more than just one of 

 these four plants. According to Strasburger's 

 theory, which is more generally accepted than 

 any other, the gametophytic generation in- 

 cludes not only all four plants with their eggs 

 and sperms, but also all preceding stages back 

 to the mother-cell. 



The water ferns (Fig. 2, B) are heterospor- 

 ous and only one of the four megaspores pro- 

 dvices a mature plant, the other three becoming 



