ORIGIN OF PRIMORDIAL GERM CELLS 491 



All these procedures are reliable and a beautiful result is usu- 

 ally obtained, especially at the periphery of a section. 



Following Meves' modification of Flemniing's fluid the iron- 

 hematoxylin stain was employed. This method was found to 

 be more reliable in the moderately young embryos — 15 somites 

 to 25 somites — than the acetic-osmic-bichromate method. 



After the 5 per cent trichloracetic acid and 5 per cent subli- 

 mate fixation iron-hematoxylin and acid fuchsin gave excellent 

 results. The cytoplasm is well preserved and the attraction- 

 sphere especially so. The centrosomes were more frequently 

 seen after this procedure than after any other. 



In all cases where embryos of relatively advanced age were 

 employed, the anterior body wall, amnion and viscera were re- 

 moved, thus exposing directly to the fixative the Wolffian body 

 and gonad. All sections were cut 4 micra thick. 



Table 1 will show at a glance the number of embryos employ- 

 ed, their age, together with the methods used in fixation and 

 staining. 



ORIGIN AND HISTORY OF THE PRIMORDIAL GERM-CELLS 



1. Structure of the germ-cells 



It will be better to describe the form and structure of the pri- 

 mordial germ-cells in the chick before proceeding to a descrip- 

 tion of their origin and history. In this way the necessity of de- 

 scribing them with each stage of development will be avoided. 

 There is all the more reason for this since more than one portrayal 

 of them would be a repetition. The change in the germ-cells, 

 from their origin to the time when they jDass into the indifferent 

 gonad, is limited to one or two structures. These variations 

 will be taken up at the stage of development at which they ap- 

 pear most evident. 



Size and shape. The most noticeable attributes of these pri- 

 mordial germ-cells, the characters, which first attract attention 

 in examining a section containing them, are their size and shape. 

 Isolated in the general mesenchymal tissue they appear immense 

 (figs, 2 and 8). Their size, which is much greater than that of 



