572 Gideon S. Dodds. 



are found. However, as will be seen later, these earlier stages 

 would probably not have added anything to the knowledge of 

 the germ-cells in these studies. 



The material was fixed in corrosive sublimate acetic; the fixa- 

 tion is good, as evidenced by the mitotic figures. The material, 

 however, was preserved in formalin, which somewhat impaired 

 the staining power of the tissues and prevented the use of certain 

 stains it was desired to use. In most respects, however, the re- 

 action to stains was satisfactory. A single vial of embryos of 

 stage 14, preserved in alcohol, gave somewhat better results. 



The stain which gave the best general results was iron-haema- 

 toxlyn, with eosin or bordeaux red as a plasma stain. Best results 

 were secured by using the 'long method' for iron-haematoxylin 

 ^twenty-four hours in 4 per cent iron-alum and an equal time in 

 the 1 per cent aqueous haematoxylin solution. A set of embryos 

 left by accident for thirty-five days in the haematoxylin solution 

 was remarkably clear for showing cell boundaries, which were very 

 indistinct in all other preparations. Delafield's haematoxylin 

 gave fair results but was not so generally satisfactory as the iron- 

 haematoxylin. 



The main part of the study was from serial transverse sections 

 6 microns in thickness. A few embryos were sectioned in 

 frontal and sagittal planes. It was found more satisfactory to 

 imbed and section the whole egg than to dissect off the embryo, 

 the yolk being so soft that it did not prevent the cutting of thin 

 sections. To allow free access to fluids, an opening was always 

 made in the egg membrane, but even with this care it was almost 

 impossible to pass from absolute alcohol to the clearing fluid 

 without considerable shrinkage and wrinkling of the egg. I do 

 not think, however, that the tissue elements of the embryo were 

 more affected by shrinkage than they would have been if the yolk 

 and egg membrane had been removed. 



2. Distribution and Migration 



A clear idea of the part of the embryo where germ-cells are 

 found can best be had from a study of cross sections. Figs. 1-11 



