456 SUMMARY OF CURRENT RESEARCHES RELATING TO 



intruders. The eggs hatch in a few days, and the vounsr (verv con- 

 spicuous pure black objects) art- watched over by the male for a few 

 days more. . 



Early Monotreme Egg.*— J. T. Wilson and J. P. Hill describe an 

 early egg of Ornithorhynchus, and discuss their previous interpretation 

 of the "primitive knot," making some amendments in their conclusions. 

 An interesting photomicrograph is given of the embryonic area. 



Primordial Germ-cells of Chick.f — Charles H. Swift finds that 

 the primordial germ-cells arise anterior and antero-lateral to the embryo 

 in a specialized region of germ-cell endoderm just at the margin of the 

 area pellucida. This region has roughly the shape of a crescent, and the 

 germ-cells arise during the primitive streak stage and until the embryo 

 has about three somites. The concavity of this crescent is towards the 

 embryo and the horns extend caudalwards on either side. 



Owing to the late appearance of the mesoderm in this region, the 

 primordial germ-cells are at first in the space between endoderm and 

 ectoderm. Subsequently, by amoeboid movements, they enter the meso- 

 derm and the incipient blood-vessels of the mesoderm. They are at 

 first carried by their own movement, and later by that of the blood, to 

 all parts of the embryo and vascular area. They remain generally 

 distributed in this way until the embryo has about twenty somites. 



In embryos with about twenty to twenty-two pairs of somites, the 

 primordial germ-cells, while generally distributed in the blood-vessels, 

 are becoming relatively more numerous in the vessels of the splanchnic 

 mesoderm. This may be a real increase, probably of a chemotactic 

 nature, exerted in the region of the future gonad. Or it may be more 

 apparent than real, a degeneration of some having occurred elsewhere. 

 At this period the great majority of the cells are found in the vessels, 

 but a few, chiefly in the splanchnic mesoderm, are present in the tissues. 

 In some cases they are present in the wall of the vessel, as if fixed in the 

 act of the leaving the vessel for the tissues. 



In embryos with about twenty-three to twenty-five pairs of somites 

 the majority of the primordial germ-cells are found in the mesodermal 

 tissue of the splanchnic mesoderm near the angle of the ccelom. The 

 embryo with twenty-five somites is the oldest in which germ-cells are 

 found in the vessels. 



Hitherto the youngest bird-embryos in which the primordial cells 

 have been described are those with twenty-two to twenty-three somites. 

 Swift has carried back the history to the primitive streak stage. 



In embryos possessing about twenty-six to twenty-nine somites the 

 primordial germ-cells are found in the splanchnic mesoderm near the 

 radix mesenterii. In embryos with thirty to thirty-three somites the 

 primordial germ-cells are in the radix mesenterii and ccelomic epithelium 

 on both sides of the ccelomic angle. They remain in this position until 

 the formation of the gonad begins, when they gradually pass into that 



organ. 



* Quart. Journ. Micr. Set, lxi. (1915) pp. 15-25 (1 pi. aud 1 fig.; 

 + Amer. Journ. Anat., xv. (1914) pp. 483-516 (15 figs.). 



