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