<)0 SUMMARY OF CURRENT RESEARCHES RELATING TO 



both a consequence and a condition of sexuality, and affords convincing 

 proof of the existence of persistent structural units, which are directly 

 responsible for the characters manifested by the developing organism. 

 The significance of reduction is in the sorting out of structural 

 entities and in the distribution of entire sets of them in the sexual cells. 

 The relatively small number of chromosomes renders it impossible to 

 regard them as structural entities, and their real importance lies in 

 their structure as similarly organised groups of chromomeres, but not 

 necessarily of the same chromomeres. It is possible that the chromo- 

 meres themselves may prove to be the structural entities of the cell. 

 The constancy in form of the chromosomes is an expression of organisa- 

 tion within the cell, not of unchanging aggregation of the same con- 

 stituents. Evidence is afforded that given a complete set of chromo- 

 somes, whether in single or in duplicate, the complete life-history may 

 be covered, and that the duplicate set arising from sexuality is merely a 

 means of producing variation. 



The primordia (structural entities), which constitute the hereditary 

 mechanism, impose the limits within which development can take place, 

 but within those limits other conditions, e.g., specific exciting substances, 

 may determine the path actually followed. 



Cytology of Pollen-mother-cells of Nymphfeacese.* — AV.Lubimenko 

 and A. Maige have made a morphological and cytological study of 

 pollen-development in the Nympkasaceas. 



The authors draw the following conclusions from their investigation:-. 

 During the prosynapsis stage, the nuclei of the pollen-mother-clls 

 increase in size, until they are 4-5 times larger than the vegetative 

 nuclei. The increase in size of the nucleus is accompanied by a 

 corresponding, but less marked, increase in the size of the cells them- 

 selves. The increase in size of the nuclei may perhaps be considered 

 as the result of delay in nuclear division. During the passage from 

 prosynapsis to synapsis there is a still greater increase in the size of the 

 nucleus in proportion to the size of the cell : this increase in size is the 

 result of enlargement of the nuclear-sac and of the nucleolar and lino- 

 chromatic masses, and is always followed by a slow enlargement of the 

 mother-cell itself. In the spireme stage the volumes of the reproductive 

 nuclei undergo a diminution in size, and are then only six times larger 

 than the vegetative nuclei. This diminution of volume corresponds to 

 a re-establishment of the normal proportions in the nuclear-sac, the 

 nucleolar and the lino-chromatic masses, and is accompanied by the 

 appearance of a well-differentiated nuclear membrane. 



The chromosomes are formed by concentration of the chromatin at 

 certain points of the spireme ; they are of various forms, and seem to be 

 composed of a varying number of small bodies. 



During the period which elapses between chromosome-formation and 

 the disappearance of the nuclear membrane, the volume of the nucleus 

 diminishes by one-half. It is probable that the entire spindle is formed 

 exclusively from nuclear substance (linin and nucleolus), and that the 

 cytoplasm has no part in its constitution. 



* Rev. Gen. Bot., xix. (1907) pp. 401-25 (5 pis.). 



