THE VITAL PHENOMENA. OF THE CELL 



215 



such as are brought aboat by the fertilisation and the maturation 

 of the ova. 



FIG. 106. jl Mature Egg of an Echinoderm, containing in its yolk a very small nucleus 

 (ek) (O. Hertwig, Embryo!., Fig. 14). B Egg of a Sea-urchin, immediately after the close 

 of fertilisation. Female pro-nucleus and male pro-nucleus have united to form the 

 cleavage nucleus (fk), which occupies the centre of a protoplasmic radiation. 



Polar differentiation consists in this, that the lighter protoplasm 

 collects at one pole, and the heavier yolk substance at the other. 

 They may be more or less sharply separated from one another. 

 For instance, sections through the eggs of Amphibians do not show 

 any striking separation, the only thing being, that in the one half 

 the yolk plates are smaller, and are separated from each other by 



.4. P. 



kb k, sell 



FIG. 108. 



V. P. 



FIG. 107. 



Fis. 107. Diagram of an Egg with the nutritive yolk in a polar position (0. Hertwig, 

 mbrye!., Fig. 3). The formative yolk constitutes at the animal pole (A, P) a germ disc 

 (fc, scli), in which the germinal vesicle (fcb) is enclosed. The nutritive yolk (>id) fills the 

 rest of the egg up to the vegetative pole (V, F;. 



FIG. 108. Egg-cell (yolk) of the Hen, taken from the ovary (O. Hertwig, EmbryoL, 

 Fig. 6 A): fc, ach germinal disc; kb germinal vesicle ; g, d yellow yolk; u-,d white yolk; 

 d, h vitelline membrane. 



