THE GENERAL CONSTITUTION OF LIVING BEINGS. 77 
the starting-point of the formation and development of 
the embryo, that is, in the ovule. We shall there point 
out these three modes at work. 
The ovule is a little globule, from one to two tenths of 
a millimetre in diameter, that is, as large as a grain of 
sand that can but just be seen. It is made up of an en- 
veloping sphere, called the vitelline membrane, in which 
is found a semi-liquid gelatinous matter, to which the 
name of vitellus* is given. The vitellus in its turn presents 
a sort of nucleus which is the germinating vesicle, or that 
of Purkinje. Thus the ovule offers to view at the outset 
the marks of a true cell, but it gains, while developing, a 
structure and dimensions that soon distinguish it from one, 
and make of it a special organ. When it has reached the 
period of maturity, the germinating vesicle disappears, and 
its substance mingles with that of the vitellus. At the 
same time the latter shrinks upon itself, and contracts. 
There comes between it and the wall of the vitelline mem- 
brane a space which fills up with a clear liquid. It is at 
this moment that the phenomenon of fecundation occurs, 
which is owing to the penetration of the spermatozoa, 
which make their way into the newly-formed space that we 
have mentioned. Then the vitellus loses shape, and for sev- 
eral minutes goes through a series of very varied gyratory 
motions, which Robin has studied. Simultaneously the 
spermatozoa—which are, as Robin has proved, true ana- 
tomical elements proceeding from male ovules, analogous 
to the embryonic cells of the female ovules— dissolve, 
and thus mingle the substance of one parent with that 
of the other, which they impregnate. There is then re- 
marked a very curious fact, also discovered and studied 
1 Birds’ eggs contain their ovule in the centre, the development of 
which, instead of taking place by means of materials provided directly 
by the mother, is made at the expense of materials contained in the egs, 
that is, the white and the yelk. 
