Reproductive Biology and Embryology of the ... - Seaturtle.org
Reproductive Biology and Embryology of the ... - Seaturtle.org
Reproductive Biology and Embryology of the ... - Seaturtle.org
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
390 REPRODUCTIVE BIOLOGY AND EMBRYOLOGY OF CROCDDILIANS<br />
C. niloticus occurs first in <strong>the</strong> middle region <strong>of</strong> <strong>the</strong> embryo (Fig. 15A), nearer<br />
<strong>the</strong> posterior end <strong>of</strong> <strong>the</strong> neural groove (Voeltzkow, 1899, 1901), but in A.<br />
mississippiensis it occurs nearer <strong>the</strong> cranial end <strong>of</strong> <strong>the</strong> neural folds (Reese,<br />
1908, 1915a).<br />
The blastoporal or neurenteric canal is visible in all early embryos until<br />
after <strong>the</strong> closure <strong>of</strong> <strong>the</strong> neural canal. As in earlier development (Fig. 12E),<br />
<strong>the</strong> blastoporal canal runs through <strong>the</strong> embryo (Fig. 14C), backward from<br />
its cranially located ventral opening [Fig. 14C (3) <strong>and</strong> (4)], to open dorsally<br />
into <strong>the</strong> neural groove at its caudal limit [Fig. 14C (5)]. Some mammals<br />
show this blastoporal canal as <strong>the</strong> chordal canal (R. Bellairs, 1971).<br />
Throughout this period, somitogenesis occurs along <strong>the</strong> median axis (Figs.<br />
15A <strong>and</strong> 17A-E), <strong>the</strong> first pair developing halfway between <strong>the</strong> anterior<br />
<strong>and</strong> posterior ends. The peripheral somitic cells are compactly arranged,<br />
but small myocoels lie in <strong>the</strong> center <strong>of</strong> <strong>the</strong> somites [Figs. 15C(i)-(v)]. The<br />
mesodermal layers cleave, forming <strong>the</strong> somatic <strong>and</strong> splanchnic components<br />
[Figs. 15C(i)-(v)] as <strong>the</strong> foregut enlarges.<br />
Early in development <strong>the</strong> head fold <strong>of</strong> <strong>the</strong> embryo projects ventrally into<br />
'<strong>the</strong> underlying yolk (Figs. l3C <strong>and</strong> D). This process is accentuated by a<br />
ventral bending <strong>of</strong> <strong>the</strong> anterior neural folds (Figs. 15B, 16A,B, <strong>and</strong> 17A-E)<br />
<strong>and</strong> still later by cranial flexure. Thus <strong>the</strong> entire cranial end <strong>of</strong> <strong>the</strong> embryo<br />
cannot be seen from above, because it is pushed down into <strong>the</strong> yolk (Figs.<br />
15B, 16A, <strong>and</strong> 17A,B). Torsion occurs between Stages 3 <strong>and</strong> 6 (Section VI),<br />
beginning anteriorly <strong>and</strong> proceeding posteriorly (Figs. 18-20). Despite earlier<br />
conflicting reports (Clarke, 1891; Reese, 1908, 1912, 1915a; Deraniyagala,<br />
1939), <strong>the</strong> embryos <strong>of</strong> Alligator mississippiensis, Crocodylus porosus, C.<br />
johnsoni, <strong>and</strong> C. niloticus usually come to lie on <strong>the</strong>ir left side (Figs. 18-20).<br />
In Crocodilians, torsion occurs at a more advanced developmental stage<br />
(Stages 3-6) than in chicks (H. H. Stages 12-15, i.e., 16-20 somites).<br />
VI.<br />
STAGES OF EMBRVONIC DEVELOPMENT [AFTER<br />
EGG LAVING]<br />
The practical importance <strong>of</strong> establishing Normal Tables <strong>of</strong> development for<br />
vertebrate embryos, <strong>and</strong> ecto<strong>the</strong>rms in particular, is discussed elsewhere<br />
(Billett, Cans, <strong>and</strong> Maderson, Chapter 1, this volume). No staging scheme<br />
exists for any crocodilian. Clarke (1891), Voeltzkow (1899), <strong>and</strong> Reese<br />
Fig. 18. Alligator mississippiellsis. Stages 1 to 4 <strong>of</strong> embryonic development. Numbers indicate<br />
<strong>the</strong> stages. See text for <strong>the</strong>ir description. 0, Dorsal view; L, lateral view; V, ventral view.<br />
Dorsal (20) <strong>and</strong> ventral (2V) views <strong>of</strong> a stage 2 embryo illustrate its attachment <strong>and</strong> vertical<br />
relationship (i.e., no body torsion) to <strong>the</strong> blastoderm. Body torsion commences at stage 4 (4V)<br />
when <strong>the</strong> cranial end has rotated; it is complete by stage 6 where a ventral view (6V) illustrates<br />
its relationship to <strong>the</strong> overlying chorion. Scale bars = 1 mm.