328 



Elementary Biology. 



nucleus transformed into the head, while the cell-protoplasm 

 becomes the vibratile tail of the sperm. 



No tubules aie formed in the ovarium ; the outer cells 

 of the germinal epithelium become transformed into the 

 ovarian wall, while the central cells become ova. The ripe 

 ovum differs from the germinal cell or primitive ovum in 

 the possession of a cell-wall, the vitelline membrane, and 

 in having a large development of oil-globules in one region, 

 the pure protoplasm (with the nucleus) tending to aggre- 

 gate towards one side of the ovum. Such an accumulation 

 of fat-granules is known as yolk, and it is much more 



FIG. 183. DEVELOPMENT OF SPERM IN THE EARTHWORM. 

 (Blomfield ) 



FIG. 184. 



SPERM OF FKOG. 



(Owen.) 



.A, spermatospore.; B. eight young spermatoblasts ; c, numerous 

 spermatobLists ; D, spermatoblasts developing into sperms. 



abundant in the ova of some forms (e.g. the fowl) than in 

 the frog. Before fertilisation takes place the nucleus of 

 the ovum undergoes karyokinesis (p. 78) and segments, one 

 half remaining as the nucleus of the ovum, the other half 

 being extruded and forming the so-called polar body (fig. 

 186). A second polar body is then extruded in the same 

 manner. Only one sperm fuses with the ovum, and its 

 nucleus (known as the male pronucleus) unites with the 

 new nucleus of the ovum (to which the term female pro- 

 nucleus has been given). When the fusion is complete the 

 ovum has been fertilised, viz. has become an embryo. Seg- 



