ORIGIN OF PRIMORDIAL GERM CELLS 493 



the neighboring cells, is due to an increase in volume of both 

 nucleus and cytoplasm. Their shape also is different from that 

 of the surrounding cells. The latter are frequently stellate, al- 

 though they may be round, oval, flattened, or elongated, while 

 the germ-cells are always round or oval (figs. 2, 6, and 10). The 

 germ-cells have an average diameter of about 16 micra. They 

 may have a diameter as small as 14 micra, while in exceptional 

 cases this may reach 20 or even 22 micra. 



Nucleus. The large nucleus, which has a diameter of 8 to 12 

 micra, is spherical and vesicular, and is nearly always eccentric- 

 ally placed (figs. 7, 8 and 13). It is surrounded by a definite 

 nuclear membrane. On comparing the nuclei of the primordial 

 germ-cells, with those of the neighboring cells, in addition to the 

 disparity in size, it is noticed that the germ-cell nuclei are clearer. 

 The germ-cell nuclei seem to contain about the same amount of 

 chromatin as the nuclei of neighboring cells, but owing to the in- 

 crease in size of the former it appears greatly reduced in amount. 

 The germ-cell nucleus, at one pole, is separated from the cell 

 membrane by a thin layer of cytoplasm, while the attraction 

 sphere generally occupies the other pole, where a wider expanse 

 of cytoplasm separates the nucleus from the membrane. The 

 clear appearance of the nucleus of the germ-cell is not confined 

 to the chick alone but is common to many species. 



The chromatin within the nucleus generally has a definite ar- 

 rangement, being grouped in two masses (figs. 2, 4, and 8). This 

 holds in nearly all stages except the youngest where the chroma- 

 tin has the appearance of a reticulum. The chromatin in the 

 two masses, mentioned above, appears in the granular form. 

 The two masses themselves may be quite distinct, being sepa- 

 rated by a wide clear interval, or they may approach each other 

 closely. In some cases they are connected by granular threads. 

 This definite arrangement of the chromatin does not appear after 

 one fixation only, but can be observed after acetic-osmic-bichro- 

 mate, equal parts of 5 per cent trichloracetic acid and 5 per cent 

 sublimate as well as Meves' modification of Flemming's fluid. 



The attraction-sphere. The attraction sphere is the most char- 

 acteristic organ of the primordial germ-cell in the chick, and. 



