ORIGIN OF DEFINITIVE SEX-CELLS 447 



is this so as regards the role played by the primordial germ-cells. 

 3) It seemed especially desirable to continue the study of the 

 germ-cells since the writer had just finished an investigation 

 dealing with the origin and early history of these cells in the chick. 



In selecting a fixative for the very young chick embryos, those 

 having fewer than 20 pairs of somites, it was found that fluids 

 containing osmic acid had a decided drawback. This was due 

 to the fat staining properties of the acid. In these very young 

 embryos the germ-cells are loaded with vitellus and when this 

 is blackened by the osmic acid it is almost impossible to see any 

 cytological detail. This property, however, becomes an asset 

 in the germ-cells in embryos of three days incubation and over, 

 since it facilitates identification. In these embryos the germ- 

 cells possess comparatively little vitellus, so that when stained 

 no detail is occluded. 



Fixatives such as Meves' modification of Flemming's fluid, 

 the acetic-osmic-bichromate mixture, and Hermann's fluid, all 

 of which contain osmic acid, were found to be best adapted for 

 this investigation, in which the embryos employed were all older 

 than three days. They preserve such cytoplasmic structures 

 as mitochondria and the attraction-sphere excellently, and 

 because of their osmic acid content, bring out the yolk spheres 

 plainly. 



In embryos between 3 and 8 days' incubation it was found that 

 the ordinary acetic-osmic-bichromate mixture was most effica- 

 cious, but in older embryos, Meves or a modified acetic-osmic- 

 bichromate fluid worked best. The latter fixative was modified 

 by using two drops of acetic acid instead of the usual one. 



There is no need of entering into detail in regard to all the 

 staining methods employed since they are described by Bensley 

 ^'11), and Cowdry ('12). However, one method has not been 

 published and I shall describe it in detail. It is called the Ben- 

 sley anilin-acid fuchsin-Wright's blood stain method. It is 

 used after the acetic-osmic-bichromate mixture preferably. The 

 pieces of tissue are imbedded and cut in the usual way. This 

 method is as follows: 



1. Pass down through toluol, absolute, 95, 70 and 50 per cent 

 alcohols into water. 



