GERM CELLS OF ANURANS 289 
No investigation of the germ-cell cycle in the Chordata should 
be regarded as complete or as being more than a half-truth 
which does not take into consideration the entire history of the 
germ-cell cycle, from the origin of the primordial germ cells to 
the formation of the definitive sexual elements of the adult. The 
investigators of the keimbahn have not gone far enough, for 
between the origin of the primordial germ cells and the formation 
of the ripe sexual products there is a critical stage in the germ-cell 
cycle, characterized by a precocious and abortive maturation, 
degeneration, and reformation of a new line of germ cells, per- 
haps by transformation of mesothelial elements, but more proba- 
bly by active mitosis of a few left-over cells of the primordial line. 
According to Hegner (Germ-cell cycle of animals, p. 99), ger- 
minal epithelium theories of germ-cell origin have Uttle if any evi- 
dence in their favor, since no one has actually observed a trans- 
formation of peritoneal or mesoblast cells into germ cells. '^On 
the other hand there is an abundance of proof that these cells 
(germ cells) migrate from some distance into the position of the 
sex glands." 
The writer is quite in agreement with Hegner regarding the 
existence of a keimbahn in vertebratqs, but is not so sure that no 
one has actually observed a transformation of mesoblast cells 
into germ cells in late stages of development or that germinal- 
epithelium theories have httle if any evidence in their favor. My 
observations on the larval bullfrog have taught me caution in 
regard to dogmatizing on this problem. Odd as it may seem, it 
is not impossible, in the hght of conditions described above for 
the bullfrog larvae, that the primordial germ cells of vertebrates, 
i.e., the keimbahn elements discussed by Hegner, may possibly be 
found upon further investigation to contribute little if any to 
the definitive sex products of the adult organism. Further inves- 
tigation of the germ-cell cycle of the chordates may possibly 
enable the 'mesodermists' to turn the tables on the 'entoder- 
mists' with a vengeance by showing that no one has actually 
observed a transformation of keimbahn cells into definitive sex 
products. Though regarding himself as an entodermist, and tak- 
ing the point of view that the keimbahn is probably continuous in 
