Reproductive Biology and Embryology of the ... - Seaturtle.org

386 REPROOUCTIVE BIOLOGY ANO EMBRYOLOGY OF CROCOOILIANS
Stages 1 and 10 (Figs. 18-20). Development of the dorsal amniotic fold
facilitates craniocaudal separation of the embryo from the blastoderm
(Figs. 15B, 16B, 17A-E, and 18), but the process is not completed caudally
until Stage 3 (Fig. 18). With further development of the blastoderm consisting
of ectoderm and endoderm (Fig. 12E), the neural groove and blastopore
become clearly demarcated (Figs. 12C-E). The endoderm may form
"tails" that extend outward and downward into the underlying yolk
(Voeltzkow, 1899, 1901; Figs. 12C-E), which probably represent presumptive
extra-embryonic endoderm. The blastopore is relatively large and penetrates
the entire blastoderm (Figs. 12C-E), and the primitive streak lies
posterior to the blastopore (Figs. 12C-E).
With the rapid delineation of the body folds (Figs. 13A and B), the
boundary between embryonic and extra-embryonic tissues becomes discernible.
Figures 13A-D show the well-defined head fold bounded anteriorly
by the proamnion. The beginning of the foregut is evident. The
notochord extends along the midline from the head fold to the anterior
edge of the blastopore (Fig. 13C). Earlier explanations (Reese, 1908, 1912,
1915a, 1926) of the origin of the notochord are dubious in the light of
current data from other vertebrates. The primitive streak and primitive
groove lie posterior to the blastopore (Figs. 13A-D); the primitive groove
[Fig. 14C (6)-(7)] is continuous with its posterior end. The ectoderm bordering
this groove is thickened, and its two elevations constitute the primitive
streak (Figs. 13A-C, 14A, B, C (6)-(7)). The primitive streak extends
about one-third the distance between the head fold and the blastopore
(Reese, 1908, 1915a). The posterior end of the neural groove is said to be
continuous with the primitive groove, and the posterior ends of the neural
folds continuous with the edges of the primitive streak; these structures
can only be demarcated arbitrarily from the dorsal opening of the blastopore
(Reese, 1908, 1912, 1915a). This type of gastrulation (blastoporal
canal, etc.) is found in all reptiles, although specific details may differ.
Neural folds have a double origin in Alligator mississippiensis (Clarke,
1891; Reese, 1908, 1912, 1915a) and Crocodlflus niloticus (Voeltzkow, 1899,
1901). The posterior folds arise as ectode~mal ridges extending forward
from the blastopore and bounding the neural groove (Figs. 13A,B, and
14A, B). However, a secondary fold occurs anteriorly in the head region
(Figs. 13A,B, and 14A,B) and grows posteriorly along the median dorsal
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