326 Papers from the Department of Marine Biology. 



The body of the 6-day embryo lies almost flat upon the yolk; the 

 gut is open behind the cephalic extremity of the pronephros. Only 

 at this point is there a mesentery, and it is very short and contains no 

 germ-cells. The germ-cells occur in both the entoderm and mesoderm 

 of the open gut, the majority lying scattered in the mesoderm medially. 



YOUNGEST EMBRYOS; FROM 2 TO 5 DAYS. 



Towards the end of the second day of incubation, when 5 somites 

 are formed, the primordial germ-cells are just becoming segregated 

 within the entoderm of the area pelllucida. The cells now occur in 

 two bilateral cords, more or less oval in transection. These cords 

 extend from just in front of the primitive pit (neurenteric canal) to the 

 caudal end of the primitive streak; they lie laterally in the area pel- 

 lucida. The germ-cells are not yet as closely aggregated as during 

 the following day. Cells can still be found in process of migration 

 from the yolk-sac entoderm of the area opaca to the entoderm of the 

 area pellucida. 



At the 3-day stage (10 somites) the cords of primordial germ-cells 

 are more closely segregated and have apparently moved slightly more 

 medially (fig. 1, plate 3). They now extend from the sixth somite 

 to the caudal extremity of the primitive streak. These cords do not 

 have a uniform diameter throughout, but are more or less interrupted, 

 suggesting a segmental condition, recalling the ^gonotome theory" 

 of Riickert. 



Figure 15, plate 2, shows a germ-cell from among the yolk-laden 

 entodermal cells of the area opaca of the 3-day stage; an entoderm- 

 cell nucleus is added at the right (a) to show the similarity of nuclear 

 condition between the germ and entodermal cells. Figure 14, plate 2, 

 shows a germ-cell taken from the area pellucida. This cell has a 

 diameter of 16.5 microns. Figure 13, plate 2, shows a similar cell in 

 process of migration from among the entodermal cells of the area 

 pellucida into the overlying visceral layer of the lateral mesoderm. 

 Closely similar conditions appear also at the 2-day stage. 



The 5-day stage (fig. 2, plate 3, and fig. 1, plate 4) represents a 

 crucial epoch in the early history of the primordial germ-cells. The 

 gut is open throughout and the embryo lies flat upon the yolk. The 

 entodermal cords of germ-cells have made an intimate linear contact 

 with the visceral plate of the lateral mesoderm (fig. 1 , plate 4) , and are 

 beginning to pass into the latter and to migrate medially. The 4-day 

 stage shows transition conditions between those described for the 3 and 

 5 day stages. Meanwhile the angioblast layer of the visceral meso- 

 derm is becoming vascularized, and an occasional cell may become in- 

 closed by, or migrate into, a blood-channel, from which it again migrates 

 into the surrounding mesenchyma or possibly later degenerates some- 

 where in the blood-vessels. 



