INTERNAL CAUSES OF VARIATION 165. 



Maturation and reduction in animals and in plants different in 

 appearance but not in fact. This remarkably significant process 

 is fundamentally the same in animals and plants, differing con- 

 siderably, however, in detail. It is simpler in animals and more 

 direct. In them the 'last two cell divisions always (apparently) 

 give rise in the male to four functional spermatozoa, but in the 

 female to one functional cell, retaining nearly all the cytoplasm, 

 and to three polar bodies incapable of fertilization and destined 

 to wither away and disappear. The same general facts seem to 

 hold for animals of all species, and Wilson remarks 1 : 



The evidence is steadily accumulating that reduction is accomplished 

 by two maturation divisions throughout the animal kingdom, even in the 

 unicellular forms ; though in certain Infusoria an additional division occurs, 

 while in some other Protozoa only one maturation division has thus far 

 been made out. 



Among plants, also, two maturation divisions occur in all the higher 

 forms, and in some at least of the lower ones. Here, however, the phe- 

 nomena are complicated by the fact that the two divisions do not, as a rule, 

 give rise directly to the four sexual germ cells, but to asexual spores which 

 undergo additional divisions before the definitive germ cells are produced. 2 

 [The end product, however, shows the same reduction in the number of 

 chromosomes.] 



A brief description of reduction in animals is worth consider- 

 ing somewhat in detail, as it is fairly well known and cannot 

 fail to impress the student with its fundamental significance and 

 the nicety of adjustment of the mechanism of living processes. 



Reduction in the female. 3 Among animals the production of 

 the female germ cell (the ovum) is the special function of the 

 ovaries. In the tissues of these organs cell division proceeds 

 under the usual mitotic plan, giving rise to a series of cells 

 known as oogonia. At a certain point mitotic division halts, and 

 each cell prepares for the final (maturation) changes. Food 

 material is absorbed, the cytoplasm increases in bulk, the nucleus 

 greatly enlarges, and the cell, now known as an oocyte, is ready 



1 Wilson, The Cell, p. 235. 



2 Ibid. pp. 235-236. Note that, in general, polar bodies are not formed in 

 plants. 



3 Ibid. pp. 236-240. This description applies to the animal. The details are 

 distinctly different in plants, to be discussed later. 



