Homepage
Login
Profile
BookClubs
dmBox
Read Ebook: Development of the Digestive Canal of the American Alligator by Reese A M Albert Moore Reese C M Illustrator
Font size:
Background color:
Text color:
Add to tbrJar First Page Next Page
Ebook has 60 lines and 14452 words, and 2 pages
Illustrator: C. M. Reese
Smithsonian Miscellaneous Collections Volume 56 Number 11
DEVELOPMENT OF THE DIGESTIVE CANAL OF THE AMERICAN ALLIGATOR
With Fifteen Plates
City of Washington Published by the Smithsonian Institution 1910
DEVELOPMENT OF THE DIGESTIVE CANAL OF THE AMERICAN ALLIGATOR
Professor of Zoology, West Virginia University
In a previous paper the writer described the general features in the development of the American Alligator; and in other papers special features were taken up in more detail.
In the present paper the development of the enteron is described in detail, but the derivatives of the digestive tract are mentioned only incidentally; the development of these latter structures may be described in a later paper.
No detailed description of the histological changes taking place during development has been attempted, though a brief description of the histology is given for each stage discussed.
The material upon which this work was done is the same as that used for the preceding researches. It was collected by the author in central Florida and southern Georgia by means of a grant from the Smithsonian Institution, for which assistance acknowledgment is herewith gratefully made.
Various methods of fixation were employed in preserving the material. In practically all cases the embryos were stained in toto with Borax Carmine and on the slide with Lyon's Blue. Transverse, sagittal, and horizontal sections were cut, their thickness varying from five to thirty microns, depending upon the size of the embryos.
The first indication of the formation of the enteron is seen in the very early embryo shown, from the dorsal aspect, in figure 1. The medullary folds and notochord are evident at this stage, but no mesoblastic somites are to be seen.
Figure 3 is a dorsal view of the next stage to be described; about fifteen pairs of somites are present.
Figure 4 is a surface view of the next stage to be described. There are here about twenty pairs of somites, though the exact number cannot be determined. Although not visible externally in the surface view shown, the gill clefts are beginning to form, and the first one opens to the exterior as will be seen in sections of another embryo of this stage. The mouth has now broken through, putting the wide pharynx into communication with the exterior; probably the mouth opening is formed at about the time of the opening of the first gill cleft.
The next stage to be studied is shown in surface view in figure 5.
Caudad to the openings of the Wolffian ducts the cloaca extends ventrad as a narrow, solid tongue of epithelium towards the exterior, figure 5I, and fuses with the superficial ectoderm at the caudal end of a prominent ridge that lies in the mid-ventral line between the posterior appendages. In this embryo the cloaca has no actual opening to the exterior; the walls of the part that projects towards the exterior are in close contact, except in the region of the openings of the Wolffian ducts, as is shown in figure 5H.
Figure 5J is a composite drawing from reconstructions of the enterons of two embryos of approximately this stage. One of these reconstructions was plotted on paper from a series of transverse sections; the other was made in wax from a series of sagittal sections. For the sake of simplicity the gill clefts are not represented, and the pharynx, mouth, and liver are represented in outline only. For the same reason the lung rudiment of one side only is shown.
The yolk-stalk, or unclosed region of the enteron, is still of considerable extent, though its exact boundaries are not easy to determine. The distance between the anterior and posterior intestinal portals is approximately shown in the figure under discussion.
The hindgut is cylindrical in cross section and of about the same diameter throughout, except for a slight enlargement in the cloacal region.
The post-anal gut is not shown here; it will be described in connection with the next reconstruction where it is figured.
Figure 6 is a surface view in profile of an embryo of the next stage to be studied. The manus and pes are here well developed, and the general development of the embryo is in considerable advancement over the last stage studied.
The transverse sections now to be described have been selected from the series from which the reconstruction, just described, was made.
As has been said, the oesophagus does not open directly into the pharynx, but is separated from it by a membrane which consists of the flattened epithelial layers of both cavities separated by a thin layer of mesoblast. This partition between the pharynx and the oesophagus is not a mere fold of mucous membrane, but is a complete, though thin, wall, easily seen in the series of sagittal sections from which this region of the embryo was drawn. The anterior end of the oesophagus is suddenly constricted so that the actual opening closed by this partition is not large.
Followed caudad the dorso-ventral diameter of the oesophagus varies somewhat, as does the lateral diameter, but it remains large throughout and opens into the stomach with no sharp line of demarkation. The character of the epithelium of the enteron caudad to the pharynx will be discussed in connection with the sections to be described below.
The ascending intestinal loop is of slightly less caliber than the descending loop above mentioned; it passes dorsal and cephalad to the posterior border of the gizzard where its lumen is continuous, for a short distance, with that of the descending loop above described. This unusual condition is probably abnormal, but owing to lack of material only one series of this stage was studied.
Eighty-five sections caudad to the one under discussion the trachea divides into the two bronchi. These bronchi gradually separate from each other until, at the point at which they open into the lungs, about eighty sections caudad to their point of separation, they lie on either side of the ventral third of the oesophagus.
Figure 7D represents a section through the plane 680 of figure 7.
Figure 7G, the last of this series, represents a section through the cloaca, caudad to the urinary openings, in the plane 1060 of figure 7. The epithelium of the cloaca is, of course, simply a continuation of that of the surface of the body, somewhat thickened, perhaps, in the deeper regions.
The mucosa of the inside of the pharynx and the anterior end of the oesophagus, exposed by the dissection, is thrown into numerous longitudinal folds, not shown in the figure; these well-marked folds extend throughout the length of the oesophagus.
Except in the enlarged region near the pylorus the lumen of the intestine is almost obliterated by the folding of its thick walls, so that little or nothing can be told of its lining with the naked eye.
A distinct mesentery holds the loops of the intestine in position and binds the entire enteron close to the dorsal body wall. Because of the lack of properly fixed tissue no sections of the enteron of this stage were made.
REFERENCES
DESCRIPTION OF FIGURES 1-8, PLATES 1-15
The surface views were drawn, under the author's direction, by Miss C. M. Reese. The first two of these views were copied, by permission, from S. F. Clarke; the others were drawn from the specimens themselves.
All of the figures of any one stage are given the same number, followed by distinguishing letters, so that it is possible to tell at a glance what figures belong together.
All of the figures except those from Clarke were drawn under a camera lucida.
Figure 1. A surface view of an embryo, from the dorsal aspect, at the beginning of the formation of the enteron.
Figure 1A. A sagittal section of an embryo of approximately the age of the one shown in figure 1. x 43.
Figure 2. A dorsal view of an embryo with five pairs of mesoblastic somites.
Figure 2A. A sagittal section of an embryo of the stage shown in figure 2. x 43.
Figure 2B. A transverse section through the headfold of an embryo of the stage shown in figure 2. x 43.
Figure 3. A dorsal view of an embryo with about fifteen pairs of somites. x 20.
Figures 3A-3D. A series of transverse sections through an embryo of the stage of the one shown in figure 3. x 43.
Figure 4. A surface view of an embryo with about twenty pairs of somites. x 15.
Figures 4A-4D. A series of transverse sections through the anterior end of an embryo of the approximate age of the one shown in figure 4. x 20.
Figures 4E and 4F. Two transverse sections through the thyroid gland of this stage; more highly magnified. x 102.
Figures 4G-4M. A series of transverse sections caudad to the preceding. Figure 4H, x 43; other figures, x 20.
Figure 5. A surface view, in profile, of an embryo at the time of the origin of the limbs. x 5.
Figure 5J. A composite drawing of reconstructions of the enterons of two embryos of the age of the one shown in figure 5. One reconstruction was in wax, from sagittal sections, the other was a plotted reconstruction from transverse sections. x 14.
Figure 6. A surface view, in profile, of an embryo with well developed manus and pes. x 5.
Add to tbrJar First Page Next Page