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

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REPRODUCTIVE BIOLOGY AND EMBRYOLOGY OF CROCODILIANS
BTAGEB OF EMBRYONIC DEVELOPMENT tAFTER EGG LAYINGJ 385
presumably to crocodilians in general. This is of greater value than speciesspecific
stages.
The approach adopted is twofold (Ferguson and Webb, in preparation).
First, a detailed morphological staging scheme related to specified incubation
conditions and chronological age is presented. Species-specific characters
are noted, but the criteria for designating particular stages are
"crocodilian" and species-independent. Second, 13 different standard
measurements, for example, total length, eye length and snout length,
which were made on embryos of all stages of Alligator mississippiensis,
Crocodylus johnsoni, and C. porosus, have been supplemented by data on
egg dimensions, weight, and the percentage of opaque eggshell banding.
Such morphometric data permit several analyses: determination of the
relationship between egg size and embryo size within and among species
(in general, larger embryos come from larger eggs and vice versa); determination
of the relationship between the percentage of opaque banding and
embryonic stage; determination of diagnostic ratios (e.g., eye length over
total length ratios are used to correct for the effect of egg size) for each
stage in each species; interspecific comparison of similar stages to determine
species-specific features (e.g., C. porosus embryos are larger and have
longer tails, C. johnsoni have larger limbs and narrower snouts, A. mississippiensis
have more abdominal yolk and retarded external genitalia); formulation
of regression equations of chronological age against particular morphometric
ratios (at different incubation temperatures and humidities) to
predict "morphological age" for any embryo of a particular species (d. rat
embryos, Ferguson, 1978a). This morphological aging is valuable when the
time interval between stages is long, due either to slow changes in the
external form (Stages 20-24) or because the embryo is fully formed and
merely enlarging in size and absorbing yolk (Stages 25-28). The two systems
are complementary, so that an embryo may be classified as "Stage 24,
morphological age 58.5 days." In general, staging by external morphological
criteria is most accurate up to Stage 20, because development is fast,
and the time intervals between stages are small. After Stage 20, the morphological
age based on morphometric ratios is most accurate for aging,
because the time intervals between stages are long, and the embryos are
large and easily measured.
In the follOWing summary of the morphological staging system (Normal
Table), no morphometric data are given. For each stage the most important
diagnostic features are listed first, and the essential features are illustrated
in Figs. 18-22. Details of important regions of various stages are illustrated
in Figs. 23-26. Some stages may be subdivided by reference to certain
criteria, for instance, Stages 1-6 may be supplemented with a somite count
(e.g., Stage 1120 s embryo), Stages 17-19 with the percentage of palatal
closure, and Stages 20-28 by morphometric ratios (e.g., yolk volume, yolk
scar width, total length) (Ferguson and Webb, in preparation). Initially, the
stages have not been separated by criteria such as somite counts, because
these vary in relation to other features, for example, some embryos develop
by enlarging existing somites before they form new ones, whereas
others may form new somites before enlargement (such variation is often
temperature dependent). The development of different structures does
vary independently (e.g., in a particular specimen branchial arch development
may lag behind that of the limbs) and appears to be marked at the
extremes of incubation temperatures. The present staging scheme is based
on examinations of approximately 1500 embryos of Alligator mississippiensis
and 300 each of Crocodylus porosus and C. johnsoni. All embryos were fixed
in 10% formal saline and photographed with oblique incident illumination
using a Wild M8 stereophotomicroscope.
Some incubation ages in days are given for embryos within each stage
(see Table VI). For Alligator mississippiensis, these ages, given for all stages,
are based on a number of standard series collected within three hours of
egg laying and artificially incubated at 30°C and approximately 90-100%
humidity. For Crocodylus johnsoni and C. porosus, the standard series were
incubated under similar conditions (Webb et aI., 1983a,e; unpublished).
Ages are not ascribed on the basis of the drawings in Webb et al. (1983a,e),
which were prepared as a field guide, but on reexamination of the original
embryos. The timing of development relative to stages is virtually identical
in all three species up to Stage 20 (i.e., during organogenesis); only later,
when growth is occurring, do species-specific differences appear. Some of
these may be due to differing incubation conditions (e.g., whether compensation
is made for metabolic heat); perhaps the ages given for A. mississippiensis
up to Stage 24 are similar in all species.
Table VI also includes "equivalence values" for each described stage to
the figures in Voeltzkow's (1899) report on Crocodylus niloticus and the
stages in Reese's (1915a) monograph on Alligator mississippiensis, which
latter included Clarke's (1891) illustrations. Some of the data in these older
works are inaccurate, even self-contradictory. In the stage descriptions
presented here, judgment of such major errors invoked the premise that
new data for A. mississippiensis, C. porosus, and C. johnsoni are more complete
and more detailed. Only limited data are available relating chronological
age and stage at temperatures other than 30°C. In A. lIlississippiensis,
increasing the incubation temperature (within viable limits) accelerates development
up to Stage 20 (organogenesis) to a much greater extent than in
later stages. This is similar to observations on birds (Romanoff, 1967),
snakes (Zehr, 1962), and turtles (Yntema, 1968; Ewert, 1979). However, it is
difficult to make precise statements regarding the effects of incubation
temperature on developmental rates because of tissue specificity, e.g.,
gonadal development is retarded at 34°C compared to stage by stage events
at 30°C.
There are also difficulties in adjusting regression equations for variations
of temperature during incubation in order to predict embryonic age (or
hatching dates) from morphometric ratios (Webb et al., 1983a,e). Generally,
development of alligator embryos up to Stage 20 proceeds approximately
1.2 to 1.8 times as fast at 34° as at 30°C, and approximately 1.6 to 2.0