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

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