128 Darwin, and after Darwin. 
purpose of effecting fertilization. This spermatozoon, 
as soon as it enters the periphery of the yolk, or cell- 
substance proper, sets up a series of remarkable 
phenomena. First, its own head. rapidly increases in 
size, and takes on the appearance of a cell-nucleus : this 
is called the male pronucleus. At the same time its 
tail begins to disappear, and the enlarged head proceeds 
to make its way directly towards the nucleus of the 
ovum which, as before stated, is now called the female 
pronucleus. The latter in its turn moves towards the 
former, and when the two meet they fuse into one 
mass, forming a new nucleus. Before the two actually 
meet, the spermatozoon has lost its tail altogether ; 
and it is noteworthy that during its passage through 
the protoplasmic cell-contents of the ovum, it appears 
to exercise upon this protoplasm an attractive in- 
fluence; for the granules of the latter in its vicinity 
dispose themselves around it in radiating lines. All 
these various phenomena are depicted in the above 
wood-cuts. (Figs. 34, 35.) 
Fertilization having been thus effected by fusion of 
the male and female pronuclei into a single (or new) 
nucleus, this latter body proceeds to exhibit compli- 
cated processes of karyokinesis, which, as before 
shown, are preliminary to nuclear division in the case 
of egg-cells. Indeed the karyokinetic process may 
begin in both the pronuclei before their junction is 
effected; and, even when their junction is effected, 
it does not appear that complete fusion of the so- 
called chromatin elements of the two pronuclei takes 
place. For the purpose of explaining what this 
means, and still more for the purpose of giving a 
general idea of the karyokinetic processes as a whole, 
