176 GERM-CELL DETERMINANTS [CH. 



cells which are visibly differentiated from the rest of the 

 embryo in the gastrula stage. Lastly, these two primitive 

 germ-cells are derived from a single cell of the 32-cell stage, 

 so that this one cell gives rise to the gonads and to nothing 

 else. To describe the origin of this cell it will be best to 

 begin with the egg at the time of fertilisation and trace its 

 development, as it has been worked out by ELPATIEWSKY 

 (1910), Miss STEVENS (1910 b) and BUCHNER (1910). At the 

 stage when the male and female pronuclei are approaching 

 conjugation, a body appears at the edge of the egg (usually 

 called ELPATIEWSKY'S "besonderer Korper"), consisting at 

 first of a mass of granules which stain deeply and which 

 later condense to a homogeneous stained mass. The origin 

 of this body is unfortunately uncertain. ELPATIEWSKY and 

 Miss STEVENS find no trace of it at an earlier stage, while 

 BUCHNER maintains that it is derived from the nucleus of an 

 accessory cell which is concerned in guiding the spermato- 

 zoon to the egg, and which, he supposes, enters the egg 

 protoplasm and gives rise to the "besonderer Korper." 

 The two former investigators, however, give good reasons 

 for believing that BUCHNER is mistaken, and the only other 

 suggestion is that the body is derived from chromatin 

 granules which are extruded from the oocyte nucleus at a 

 rather earlier period, although, if this is so, the intervening 

 stages have not been found. Whatever its origin, the 

 "besonderer Korper" does not divide in any of the first 

 five segmentation divisions of the egg, with the result that 

 at the 32-cell stage it is confined to one cell, and this be- 

 comes the primitive germ-cell (PL XXII, upper four figures). 

 When this cell divides, the body breaks up into several 

 pieces, some of which pass into both daughter-cells, but 

 according to ELPATIEWSKY and to Miss STEVENS the 

 distribution is uneven, one cell receiving more than the 



