570 OOGENESIS CHAP, xn 



These, in the egg-mother-cells, become arranged in pairs, 

 and each splits longitudinally, so that six tetrads are 

 formed. The nucleus, which thus contains double the 

 normal number of chromosomes, loses its membrane, 

 travels to the surface of the egg, and a spindle is formed. 

 Next the protoplasm grows out into a small projection 

 or bud, into which one end of the spindle projects 

 (Fig. 151, A). Nuclear division then takes place, without 

 a splitting of the chromosomes, one of the daughter nuclei 

 remaining in the bud (pol), the other in the ovum itself, 

 each of these nuclei containing twelve chromosomes. 

 Then follows, as usual, a division of the protoplasm, and 

 the bud becomes separated as a small cell distinguished 

 as ihe first polar cell, 



In some cases development from an unfertilised female 

 gamete takes place, the process which occurs in various 

 invertebrate groups and is not uncommon during a greater 

 part of the year among insects (e.g. the common little green 

 plant-louse or Aphis) and crustaceans (e.g. water-fleas) 

 being distinguished as parthenogenesis. It has been shown 

 that in many such cases the egg begins to develop after the 

 formation of the first polar cell, when the ovum contains the 

 number of chromosomes normal to the species. 



As a general rule, development takes place only after 

 fertilisation, and a second polar cell (B, pol) is formed 

 from the ovum, a reducing division occurring just as in 

 the case of the sperm-cells (p. 568), so that the ovum 

 and the second polar cell each contain only six chromo- 

 somes. Thus the ovum has now lost a portion of its 

 protoplasm together with three-fourths of its chromatin, 

 half having passed into the first polar cell and half of 

 what remained into the second : the remaining one- 

 fourth of the chromatin becomes enclosed in a nucleus 

 which is distinguished as the female pronucleiis (B, 

 ? pro n) . The first polar body has in many cases been 



