26 THE STRUCTURE OF CELLS 



plant cell. The latter hypothesis would be difficult to sustain in 

 the absence of a series of forms through which their evolution 

 might be traced, whilst, on the other hand, the symbiotic relation- 

 ship existing between fungi and algal cells in a lichen strongly 

 supports the presumption that an analogous case is furnished by the 

 Zoochlorella organism and its host. 



" Reduction Divisions" 



Few cytological discoveries have aroused more widespread 

 interest than that of the periodical recurrence of the so-called 

 " Reduction Divisions," which are intercalated at some point in the 

 cell-generations intervening between two consecutive sexual unions. 

 Each uniting gamete or sexual cell contains in its nucleus only 

 half the number of the chromosomes that will be characteristic of 

 the embryo resulting from their fusion, and will be retained 

 throughout its cell -generations up to those which lead in their 

 turn, more or less directly, to the production of spermatozoa and 

 mature ova. This remarkable phenomenon has been observed in 

 all the animals and plants which have been carefully studied, with 

 the exception of the more lowly or primitive forms in which the 

 nuclear history is but imperfectly understood. 



The phenomenon in question was first made known by the 

 investigations of Van Beneden in 1883 and 1887 on Ascaris. The 

 choice of this animal was in some respects perhaps not very 

 fortunate, since it does not exhibit the process in a very typical, 

 but rather in an extreme, form, and thus a certain amount of 

 misapprehension prevailed at one time respecting it. Since that 

 period, however, very numerous animals and plants have been 

 studied, with the result that the phenomenon is proved to be of 

 very general occurrence, though differing considerably in detail in 

 the various organisms. 



At first, and perhaps naturally, the view was advanced that the 

 reduced number was secured through the mere degeneration and 

 consequent elimination of the superfluous chromosomes, but it gradu- 

 ally became clear that the evidence was entirely opposed to such a 

 simple explanation, and that, on the contrary, the reduction was 

 only arrived at after an exceedingly complex rearrangement of the 

 nuclear constituents. It would, however, be going too far, as will 

 subsequently appear, to deny that any nuclear substance is lost : 

 all that can be said is that it is certain that no chromosomes, as such, 

 are normally eliminated. 



In attempting to trace the sequence of events, it must be borne 

 in mind that the process is evidently one of the highest importance, 

 seeing that it occurs alike in animals and in plants, and this 

 importance is increased rather than lessened by the further 



