NA TURE 



61 



THURSDAY, NOVEMBER iS, 1909. 



CYTOLOGICAL ASPECTS OF CERTAIN 

 BIOLOGICAL PROBLEMS, 

 y. ilpiiiikt dcr Bestiinmiing dcs Geschlechfs, Apo- 

 i;ainie, Partlicnogciicsis, uiid Reduktiouslciluiig. 

 By E. Strasburger. Heft VH., Der Histologischen 

 Beitriige. Pp. xvi + 124. (Jena: Gustav Fischer, 

 1909.) Price 6.50 marks. 



IN" the se\-enth part of his " Histologische Bcitriige," 

 Prof. Strasburger records and discusses the results 

 of investigations on a number of cytological problems 

 cliiefiy related to the nature and distribution of sex. 

 The reader will recognise the facile grasp of wide 

 and complex series of facts, as well as the lucid 

 presentation of the conclusions, which form so 

 strongly marked a feature of the works of this great 

 investigator. 



The opening chapter is devoted to an account of 

 studies on the sexual differentiation in the spores of 

 the dioecious liverwort Sphasrocarpus. The result of 

 cultures of the spores of this plant brings out the 

 remarkable fact that on the average two spores in 

 each tetrad give rise lo male, and the two other ones 

 to female individuals. This distribution does not 

 occur invariably, but it does so in the great majority 

 of cases, and the discovery is of considerable import- 

 ance and interest. It is clearly in harmony with, and 

 gives greater precision to, the observations already 

 made by others on the distribution of sex in other 

 liverworts and mosses. In the case of the plant in 

 question, it indicates that the sex of the individual to 

 which the spore will give rise is predestined during 

 the meiotic divisions of the spore mother cell — the 

 divisions in which we have good grounds for supposing 

 that many other characters are segregated and dis- 

 tiibuted. It will be remembered that it is at nieiosis 

 that the nuclear chromosomes are distributed between 

 the nuclei of two daughter nuclei, so that their number 

 in each of these nuclei is consequently halved. But 

 the coincidence needs to be utilised with caution, and 

 Prof. Strasburger himself fully discusses the diffi- 

 culties in the way of generalising too freely. 



It is obviously necessary to distinguish clearly 

 between the characters appropriate to the se.xual cells 

 (or to the gametophytes) themselves, and those sexual 

 tendencies of which they are the bearers. The latter, 

 of course, only declare themselves after fertilisation, 

 when the sex of the particular individual of the next 

 generation which springs from them is declared. 

 Whilst it is quite possible, and in certain cases it can 

 be very plausibly argued, that these sexual tendencies 

 are decided at meiosis, it appears to be quite certain 

 that the sexual characters must be regarded from a dif- 

 ferent point of view. Thus, notwithstanding the case 

 of Sphcerocarpus, where meiosis seems to determine 

 the segregation of the resulting gametophytes into a 

 pair of male and a pair of female plants respectively, a 

 consideration of any heterosporous plant — as, for 

 example, selaginella, suffices to remind one that 

 here, at any rate, the sexual character of the gameto- 

 phyte is determined by causes which operate 

 before meiosis supervenes. For the difference can be 

 XO. 20gO, VOL. 82] 



detected in the sporangia. Still more clearly is the 

 same thing apparent in dioecious plants, where the 

 male and female individuals differ from each other, 

 and thus the character of the gametophytes to which 

 they will severally give rise can be foretold at a 

 much earlier stage in the life-history. 



With regard to the segregation of the sexual ten- 

 dencies the matter is otherwise. Although the 

 questions herein involved are very complex, the 

 evidence at present available seems to indicate that the 

 segregation of the opposing sexual tendencies, where 

 it occurs at all, is achieved at the stage and by means 

 of the mechanism of meiosis. The fact that in the 

 interpretations of sex, from this point of view, there 

 exists a lack of unanimity as to the precise constitu- 

 tion of the male and female gametes does not 

 materially affect the position. Thus the question as 

 to whether maleness and femaleness can be regarded 

 as allelomorphic (or alternative, as regards the consti- 

 tution of the gametes) characters is independent of the 

 views taken as to the heterozygous character, for 

 e.xample, of the male or the female, and it may turn 

 out ultimately that different organisms behave 

 differently in this respect. In this connection it may 

 be mentioned that the alga Chara crinita produces 

 only female plants from its parthenogenetically de- 

 veloping eggs, which contain the half number of 

 chromosomes (haploid), whilst in ants, bees, and 

 wasps the corresponding unfertilised, parthenogenetic- 

 ally developing eggs invariably give rise to males. 



Intimately bound up with the question of sex are 

 the various instances of eggs which are able to 

 develop into a new individual in the absence of the 

 normal union with a sperm. Strasburger rightly, as 

 we think, criticises those who would include all these 

 cases under the term parthenogenesis. Examples are 

 known of eggs which possess the full premeiotic 

 number of chromosomes owing to the obliteration of 

 the meiotic phase from the life-history. Fertilisation 

 is always absent in such cases, but it is misleading 

 to speak of this as a parthenogenetic development, 

 seeing that the nuclear constitution of such an 

 egg deviates so fundamentally from that of a normal 

 one. Possibly the term " diploid parthenogenesis " 

 might be employed to meet such cases, though we 

 confess to a preference for the word '-parthen- 

 apogamy" as being more characteristic. 



In dealing with the general question of apogamy, 

 the author emphasises the suggestive fact that species 

 which exhibit this feature commonly possess far larger 

 numbers of chromosomes than allied and normal 

 species, and also that they are characterised by a 

 relatively high degree of variability. These two points 

 appear to us to be highly significant, and to merit a 

 closer attention than they have generally received. 



Prof. Strasburger holds stoutly to his views as to 

 the omnipotence of the nucleus in determining the 

 course of development. That is to say, the nucleus is 

 regarded as alone bearing the hereditary substance 

 which he believes to consist of distinct bodies re- 

 sponsible for the characters manifested by an indi- 

 vidual. He adheres strongly to the view, advanced 

 by Gregoire, that homologous chromosomes {i.e. 

 derived from the male and female parent respectively) 



D 



