178 GERM-CELL DETERMINANTS [CH. 



place, one of the four nuclei produced enters the pole-plasm 

 at the posterior end of the egg, and in the next division its 

 behaviour differs from that of the other three (PI. XXIII). In 

 the division of the three nuclei in the yolk a process of chroma- 

 tin-diminution takes place. Parts of each chromosome, or 

 possibly some complete chromosomes the descriptions are 

 not explicit on this point are left behind in the equatorial 

 region of the spindle in the anaphase, so that the daughter- 

 nuclei contain only a portion of the chromatin that was 

 present in the parent nucleus. The fourth nucleus, situated 

 in the pole-plasm, meanwhile undergoes a normal mitotic 

 division without diminution, and of the two nuclei pro- 

 duced from it one remains in the pole-plasm and the other 

 moves forward into the yolk. At the next (fourth) seg- 

 mentation division each of the nuclei in the yolk, including 

 the sister-nucleus of that in the pole-plasm, again undergoes 

 chromatin-diminution, and according to KAHLE in their sub- 

 sequent divisions these nuclei show only half the number of 

 chromosomes that are present in the germ-cells. At the 

 stage when eight nuclei are present the pole-plasm with its 

 included nucleus is cut off from the rest of the egg as a 

 separate cell, which does not divide again immediately. 

 This is the primitive germ-cell, and in its subsequent 

 divisions it always retains the full amount of chromatin. 

 It later divides into two, and then into four and eight, and 

 these eight cells group themselves in two rows of four, one 

 on each side of the embryo, to give rise to the ovaries. 

 Then by three more divisions the number of cells in each 

 ovary is increased to thirty-two, which is the number of 

 oocytes in the mature ovary. It is remarkable that the 

 primitive germ-cell gives rise to nothing but germ-cells, for 

 the nurse-cells originate from the enclosing mesodermal 

 envelope; also that the total number of cell-divisions from 



