1 82 GERM-CELL DETERMINANTS [CH. 



been observed in several Entomostraca, and here also germ- 

 cell determinants of very different types have been de- 

 scribed. In Cyclops and other Copepods AMMA, following up 

 the work of earlier observers, describes a cloud of staining 

 granules, which he calls "ectosomes" at one pole of the first 

 segmentation spindle of the egg ( PL XXI I , lower five figures ). 

 In Diaptomus they appear when the spindle is forming and 

 the sperm and egg nuclei have not yet united, in other 

 genera they appear first during the mitosis. They all become 

 included in one daughter-cell, and at the next division, in 

 addition to the remains of the old granules, a new lot 

 appear at one pole and are included again in one cell. This 

 is repeated till the fourth division of the granule-bearing 

 cell but before the fifth division of this cell, which is 

 simultaneous with the sixth of the other cells, the granules 

 become evenly spread through the cell, and are distributed 

 equally to both daughter-cells, which become the primitive 

 germ-cells. With regard to the origin of the "ectosomes," 

 AMMA rejects the hypothesis that they are derived from the 

 nucleus or nucleolus, and concludes that they are products 

 of the cell metabolism which appear temporarily and are 

 then re-absorbed. 



In the Cladoceran Polyphemus, KUHN (1913) finds that 

 a nurse-cell enters the egg before cleavage; in the fourth 

 segmentation division the blastomere containing it divides 

 into two cells, of which the one which includes the now frag- 

 mented nurse-cell becomes the primitive germ-cell and the 

 other the first endoderm cell. In the succeeding divisions 

 of the primitive germ-cell, the remains of the nurse-cell are 

 distributed to both daughter-cells. In other Cladocera which 

 have been examined for comparison no similar germ-cell 

 determinant has been discovered. 



From the examples described it will be seen that the so- 



