Gametogenesis, Fertilization and Parthenogenesis 



171 



vides (without diminution) into a somatic 

 cell (that undergoes diminution at its next 

 division) and the definitive primordial germ 

 cell, P4. 



(Boveri, '10a; Hogue, '10). The centrifugation 

 experiments provided the more direct evi- 

 dence. In many of the centrifuged eggs (as 

 high as 36 per cent in some experiments) it 



veg. 



veg./( 



Fig. 45. Cleavage of Ascaris megalocephala {Parascaris equorum), illustrating location of the stem cells 

 (P1-P4) which do not undergo chromatin diminution and from which the germ cells are derived. Note bend- 

 ing of the original egg axis as a result of the shifting of the blastomeres in the 4-cell stage. A, 2-cell stage; 

 B, C, D, 4-cell stage; E, 7-cell stage; F, 24-cell stage; an., animal pole; veg., vegetal pole. (From von Ubisch, 

 '43, after Boveri, '99.) 



Fig. 46. Second (A) and third (B and C) cleavages of Ascaris eggs that had been centrifuged before the 

 first cleavage; showing (in B and C) chromatin diminution in the upper cells but not in the two lower cells 

 (from Hogue, '10). 



Boveri surmised that differences in the 

 cytoplasm of different regions of the egg 

 were responsible for the occurrence or non- 

 occurrence of the diminution process and, 

 at the same time, for the determination of 

 the germ cells. This view was supported by 

 the results of studies on polyspermic eggs 

 (Boveri, '04, '10b) and on centrifuged eggs 



was found that two stem cells formed instead 

 of the single one that normally occurs. Fig- 

 ure 46 illustrates such eggs at the second and 

 third cleavages. Diminution is seen in the 

 upper two blastomeres (adjacent to an ex- 

 truded ball of cytoplasmic material) but not 

 in the two lower cells. This effect is inter- 

 preted as being due to a shift in direction 



