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a) Earlier Stages of Amnion. 
1. Trionyx and Clemmys are essentially alike in earlier stages. 
2. In an early stage (Diag. I), there is in the anterior part of 
the blastoderm a deep horse-shoe shaped 
groove, „die vordere Grenzfurche“, caused 
by the heavy head-end sinking. The poste- 
rior wall of this groove is formed by the 
head of the embryo, and the anterior wall 
is the first rudiment of the anterior fold 
of the amnion. The amnion at this part 
and stage consists only of the epiblast and hypoblast; hence it is of 
the nature of the proamnion. 
3. The whole amniotie sac is produced solely by the growth 
backward of this anterior fold in conjunction with the lateral folds 
which arise gradually from before backward. The backward shifting 
edge of the amniotic hood presents always a horse-shoe shaped outline. 
4, In a stage with 2—3 mesoblastic somites, when the amniotic 
hood has covered the anterior half of the embryo (Clemmys, 
Diags. II and II‘), the amnion consists, in the region of the sunken 
head, of the epiblast and hypoblast, and in the dorsal region, of a 
solid sheet of the epiblast alone. The mesoblast as 
yet has no share whatever in any part of the amnion 
(Diag. IT’). 
5. In a stage with 6—7 mesoblastic somites when the amnion 
has extended to the posterior end of the embryo, leaving only the 
region round the neurenteric canal exposed (Diags. III and III’), the 
