338 Papers from the Department of Marine Biology. 



more or less plausible, but can not be said to be wholly satisfactory. 

 The condition in the lizard may be simply an exaggeration of that in 

 other forms where a certain number of germ-cells go astray during the 

 period of migration. 



Since germ-cells can migrate by their own activity, it is not difficult 

 to conceive of their presence anywhere, even in the ectoderm, and 

 certainly not in the developing Wolffian duct, where, unless they 

 retrace their course towards the genital gland, they may undergo 

 degeneration (or possibly lie dormant until a favorable influence may 

 excite to the production of a tumor or a teratoma), but that they 

 should contribute to the formation of the Wolffian duct is thus far an 

 anomalous phenomenon. 



When one considers in common conditions in the chick, lizard, 

 toads and frogs, salamanders, and turtles, it becomes clear that there is 

 great variation in the manner in which the comparable elements (the 

 ''primordial germ-cells") arise, migrate to the sexual gland, and 

 develop. Underlying these discrepant phenomena, however, there 

 appears a fundamental harmony, namely, an early segregation of 

 germ-cells from somatic cells, a germinal path from blastomeres to 

 genital cells of the genital gland, probably from blastomeres to sperm 

 and ova ; a migration from the entoderm (or its derivative, the meso- 

 derm, in urodeles and lamprey), through the splanchnic layer of the 

 mesoderm to the mesentery and thence to the gonads (with possible 

 aberrant migrants anywhere). 



And in the light of Swift's latest researches in the case of the sex- 

 cords and the ova of the chick ovary and the spermatozoa of the 

 testis, it now seems clear (notwithstanding the contradictory findings 

 of Dustin in Chrysemys and of Gatenby in the frog) that the definitive 

 germ-cells have had an extra-regional origin and do not arise by 

 differentiation from germinal epithelium (coelomic epithelium), but 

 become involved in columnar invaginations of these cells to form sex- 

 cords. Certain observations (e. g., those of Felix, Dustin, and Firket) 

 to the effect that the primordial germ-cells ("primary genital cells") 

 degenerate early and are replaced by coelomic derivatives in the genital 

 gland, may possibly be explained by the facts that in certain amniotic 

 forms the primordial germ-ceils degenerate relatively more extensively 

 and that mitosis does not generally appear until the remainder reach the 

 sex-glands, v/here it becomes active in those cells involved in the sex- 

 cords formed by the "germinal epithelium." The great proliferative 

 activity of the remaining primordial germ-cells in the formation of oogo- 

 nia and the spermogonia, among the cells of the coelomic epithefium, 

 gives the appearance of an active differentiation and derivation from the 

 coelomic epithehal-cells. The products of this intense proliferation are 

 relatively smaller than the earlier germ-cell progenitors, which phenom- 

 enon makes the superficial resemblance between growing coelomic epi- 



