THE STRUCTURE OF CELLS 37 



number, which may be very considerable, of cell-generations inter- 

 venes between the division in question and the differentiation of the 

 sexual cells. It is true that, as in some of the flowering plants 

 (the embryosac development of the lily, for example), the divisions 

 giving rise to the four spores may be omitted, but the characteristic 

 features of the heterotype and homotype mitosis reappear, although 

 thus postponed, in the first divisions of the nucleus of the embryosac 

 (macrospore). This indeed is a fact of the utmost importance, as 

 emphasising the physiological necessity of the process ; just as it in 

 all probability (from its community to animals and plants) pre- 

 ceded current morphological differentiation, so now if necessary it 

 can override morphological limitations, or at any rate it is not bound 

 up with them. 



Amongst the lower plants the facts have been tolerably com- 

 pletely elucidated in the case of Fucus, an alga in which asexual 

 spore -formation does not occur. The nuclei of the plant possess 

 the double (somatic) number of chromosomes until the formation of 

 the sexual organs. The oogonium gives rise to oospheres (typically 

 eight, though some may degenerate) by these nuclear divisions. 

 Of these, the first two are respectively heterotype and homotype, 

 and follow on each other with great rapidity, the last mitosis not 

 occurring till after an interval of rest. 



In some of the desmids, and probably also in Spirogyra, there is 

 evidence to show that the reduction divisions, on the other' hand, 

 occur not at the close, but at the beginning of the life-history, with 

 the first segmentation of the fertilised oosphere. But in the 

 majority of these lower organisms information of a precise char- 

 acter is still lacking on the matter. And until our knowledge of 

 the corresponding processes in the lower animals and plants becomes, 

 much more complete than it is at present, we can scarcely expect to 

 solve the problem as to the utility or the necessity of the complex 

 events connected with the reduction divisions. 



Although the higher animals and plants exhibit considerable 

 diversity amongst themselves in the series of changes passed 

 through by the nucleus in division, as well as in the relationships exist- 

 ing between the cytoplasm on the one hand and the cell-wall on the 

 other, they nevertheless agree for the most part in the broader out- 

 lines. The points of similarity are, on the whole, more striking 

 than are the differences, and the latter can often be referred with- 

 out much difficulty to unimportant deviations from a common ground- 

 plan. But although- this is the case, the actual meaning of the 

 phenomena, as well as their phylogenetic origin (if there be one), 

 can hardly be grasped or explained by a reference to these forms 

 alone. It is in the study of the lowest forms of life that the key 

 to the solution of cytological problems may be sought for with the 

 greatest hope of success, for amidst the striking diversity exhibited 



