No. 463.] STUDIES ON PLANT CELL.— VI. 451 
possibilities in the thallophytes which must remain as specula- 
tions until investigations have advanced much farther in this 
difficult field of cell study. The basis of any theories at present 
must be phylogenetic, a principle that has not been followed in 
some of the work upon the thallophytes. 
Gametogenesis in plants is full of interest TNNT of the 
sharp differences from the processes of spermatogenesis and 
oógenesis in animals. In animals the period of gametogenesis 
is one of unusual activity. After the germ cells are differenti- 
ated there follows a period of cell growth, with the peculiar 
activity termed synapsis, during which the number of chromo- 
somes is reduced to one half the number characteristic of the 
species. The germ cells emerge from the growth periods as 
primary spermatocytes or oócytes which give rise respectively 
by two successive mitoses to four spermatids or to an egg with 
its accompanying polar bodies. The gametes have one half the 
number of chromosomes characteristic of the species, so that 
the period of gametogenesis is one of chromosome reduction. 
The character of this process of reduction will be considered 
when we take up the analogous phenomena in plants after the 
discussion of sporogenesis. Gametogenesis in plants is in strik- 
ing contrast to that in animals. In all higher groups (those 
above the thallophytes) we know that the gametes have the 
same number of chromosomes as the vegetative cells of the 
parent plant (gametophyte). There is no reduction of the chro- 
mosomes at the time of gametogenesis, that phenomenon taking 
place at the end of the sporophyte generation with sporogenesis. 
Also, there are no peculiarities of the mitoses immediately 
preceding gametogenesis excepting such as concern the devel- 
opment of cilia-bearing organs (blepharoplasts) or slight pecul- 
iarities in the form or size of the spindles, for such nuclear 
figures are frequently different in these particulars from the 
mitoses in vegetative cells of the gametophyte. The differences 
concern chiefly the structure of the sperm, and have been de- 
scribed in our account of that structure (Amer. Nat., vol. 38, 
July and August, p. 576. 1904). 
To Strasburger above all others should be given the credit 
of making clear these important characteristics of gametogene- 
