ZOOLOGY: H. E. JORDAN 
273 
as representatives of a dilatory process of mesoderm derivation from 
entoderm. 
As regards amphibia, the later observations incline AUen^^ to con- 
clude that in anurans the germ cells have an entodermal origin (i.e., 
original segregation), in urodeles a mesodermal origin. 
The germ-cell history of Caretta is very similar to that first described 
by Allen^^ for Chrysemys marginata — and more recently confirmed by 
Dustin^^ —and to that described by Woods^^ for the dogfish. This 
may be summarized as follows: 
1. The primordial germ cells in Caretta migrate during the second 
day (5 somites; 2 mm. length) from the yolk-sac entoderm, where 
they were more or less widely scattered caudally, into the lateral bor- 
der of the area pellucida on each side of the embryonic disc. Here 
they become sharply segregated by the beginning of the third day 
(10 somites; 3 mm. length) into bilateral cords situated in the ento- 
derm of the area pellucida laterally, in the caudal half of the disc. In 
the two-day embryo they extend from the neurenteric canal to the 
end of the primitive streak; in the three-day embryo from the sixth 
somite to the caudal extremity of the streak. The cords become more 
medially placed, make a linear connection with the overlying visceral 
mesoderm, and their cells migrate during the fifth day into this meso- 
derm, and thence medially (during the sixth and seventh days) towards 
the root of the forming mesentery of the closing hind-gut. Individual 
cells migrate medially also within, or back into, the entoderm of the 
gut. The germ cells in the medial entoderm become included in the 
tunica mucosa of the closed hind-gut, those in the mesoderm in the 
enveloping mesenchyma and the gut end of the mesentery. From 
these locations the majority of the germ cells subsequently (seventh 
to twelfth day) migrate up the mesentery and across the celomic angle 
into the future sexual gland. They become incorporated among the 
mesenchymal cells of the gland and the covering peritoneal epitheHum, 
where they suffer no striking change in form, size or content at least 
as late as the thirty- second day of incubation. 
2. The germ cells migrate by ameboid activity, assisted in small 
part probably by the factor of unequal growth, involving the shift- 
ing of the medial portion of the splanchnopleure to the mesentery, 
and the dorsal portion of the mesentery to the gonads. 
3. The migration period is not sharply limited. It is at its height 
from the seventh to the twelfth day, and practically ceases about the 
sixteenth day. But occasional extra-regional cells may still be found 
in the gut and mesentery at the thirty-second day stage, usually, how- 
ever, showing signs of degeneration. 
