78 NATURE AND LIFE. 
by Robin, the production of polar globules. These glob- 
ules are little prominences which rise by gemmation on 
the surface of the vitellus. They mark the point at which 
the depression of the latter, and then its breaking up, will 
afterward begin. At the same time a new nucleus, the 
vitelline nucleus, is born complete, by spontaneous gen- 
eration, in the depths of the primitive mass. This nucleus 
breaks up and divides into several nuclei, about which the 
substance of the vitellus forms separate groups, and there 
thus arise cells which proceed, by ranging themselves close 
against the wall of the vitelline membrane, to form another 
membrane, called the blastoderma. This segmentation of 
the vitellus, discovered in 1824 by Prévost and Dumas, is 
exceedingly important, seeing that the first elements of the 
embryo proceed immediately from blastodermic cells. It 
must be remarked that in insects and spiders, as Robin 
has discovered, the vitellus does not split up. In these 
little beings the cells of the blastoderma are formed by 
gemmation of the surface portion of the vitellus; that is 
to say, the polar globules, instead of being developed at 
one single point of the latter, make their appearance over 
its whole surface, to compose the blastodermic membrane. 
In brief, the essential mechanism of generation is reduced 
to the following succession of phenomena, taking place in 
the depths of the ovule or of the egg within a time which 
varies from twelve to twenty-four hours: 1. The disappear- 
ance of the germinating vesicle; 2. Shrinking of the vi- 
tellus; 3. Penetration by the spermatozoa; 4. Loss of form 
and gyration by the vitellus; 5. Production of the polar 
globules by gemmation; 6. Origin of the vitelline nucleus 
by genesis; 7. Splitting up of the vitellus; 8. Composition 
of the blastoderma; 9. Formation of the embryonic dot; 
10. Appearance of the first definite elements of the embryo. 
As we see, the new being, formed of well-constituted ana- 
tomical elements, has received none of them from its 
