ORIGIN OF PRIMORDIAL GERM CELLS 487 



an amoeboid form and a lateral position in the hypoblast; in 

 older stages these cells still have an amoeboid form, but have a 

 more median position, which they have attained by a centripetal 

 movement along the hypoblast. In the still older embryos these 

 cells are found to have lost their amoeboid character and to be 

 in the entoderm under the notochord or in the mesoderm in that 

 immediate neighborhood. Thence, according to the evidence of 

 the still older embryos, they pass into the mesentery and finally 

 assume a position which corresponds to that of the future sex- 

 gland. 



Woods ('02), working on Acanthias, found that the primordial 

 germ-cells were first evident in the entoderm and in the periblast 

 and thence migrated to the sex-gland anlage. 



Jarvis ('08), in Phrynosoma, found that the germ-cells ap- 

 peared first in the entoderm of the vascular area of the blastoderm, 

 where they were cephalad, caudad, and laterad to the embryo. 

 Then by a definite migration path they reached the germinal 

 anlage. 



Dodds ('10), investigating the question of germ origin and his- 

 tor}^ in the teleost, Lophius, was able to recognize the primordial 

 germ-cells in the primitive entoblast, when the blastoderm had 

 not quite half covered the yolk. He, however, believed that 

 they were set apart at a time earlier still in embryonic history. 



Finally, as far as vertebrates are concerned, Fuss ('11), in a 

 human embryo of four weeks, with about 33 somites, demon- 

 strated extra-regional primordial germ-cells in the mesentery di- 

 rectly under the coelomic epithelium. 



Among invertebrates, in many phyla, the primordial germ- 

 cells have been observed at a distance from the site of the future 

 sex-gland, that is, in an extra-regional position, and in several 

 cases have been traced back to their true origin. Indeed, Bal- 

 biani's ('85) work with Chironomus was one of the very earliest 

 in which a. definite origin was proven for the primordial germ- 

 cells. He found that they originated from cells differentiated 

 very early in the history of the segmenting egg. 



The most remarkable known case of early differentiation of 

 the primordial germ-cells is that of Ascaris, in which Boveri 



