486 CHARLES H. SWIFT 



plate of mesoderm, in the region between splanchnic mesoderm 

 and entoderm and in the entoderm. 



Eigenmann ('97) was one of the first to prove that primordial 

 germ-cells may have an extra-regional origin. His work on the 

 \dviparous teleost Cymatogaster showed that the primordial germ- 

 cells in that form, have an origin from cells having the size of 

 fifth cleavage blastomeres. 



Beard ('04) discovered in Raja batis that the primordial germ- 

 cells have an origin from a group of cells in the anterior part of 

 the blastoderm, beneath the embryonic anlage, and that these 

 cells then migrate in towards the region of the future sex-glands 

 along a very definite path, through the hypoblast of the gut and 

 the line between hypoblast and unsegmented splanchnic meso- 

 derm. These early extra-embryonic germ-cells, which he called 

 megaspheres, differ in no particular, except size, from the germ- 

 cells, which are found in the older embiyos. 



Nussbaum ('01), Rubaschkin ('07), and von Berenberg-Gossler 

 ('12), in addition to Hoffmann ('93) have investigated the pri- 

 mordial germ-cells in bird embryos. The first three worked with 

 the chick and were able to find in embryos of 22 to 23 somites 

 typical germ-cells in the entoderm and splanchnic mesoderm lat- 

 eral to the coelomic angle, some of which in older embryos passed 

 into the germinal epithelium. The 22-somite embryo is the 

 youngest in which primordial germ-cells haA^e been found in the 

 chick. 



Allen ('06), ('09), ('09), investigated this question of primordial 

 germ-cell origin in Chrysemys, Amia and Lepidosteus and dis- 

 covered that the primordial germ-cells in these forms had an en- 

 todermal origin, in the case of Chrysemys from the entoderm 

 near the margin of the area pellucida in a zone extending on 

 either side from the anterior extremity of the pronephros to a 

 point behind the embryo. The primordial germ-cells of Lepi- 

 dosteus have a not very different origin, while in Amia they were 

 shown to come from the entoderm of the roof and margin of the 

 floor of the sub-germinal cavity. 



Allen has shown that in these forms there is a definite migra- 

 tion. In the youngest embryos the primordial germ-cells have 



