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 thore 

 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 Prevost 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 



