Albert M. Reese

“ The outlines of the middle and extreme posterior regions of the ectoblast are much more irregular and ragged than is shown in the figure. The plane of the section passes through the notochord (nt) in the posterior region, but not in the anterior end of the embryo, where a layer of mesoblast (mes) is seen. The great size of the blastopore (blp) is well shown, as is the beginning of the foregut (fg) . Comparison of this figure with the more anterior transverse sections and with the dorsal surface view of this stage will make the rather unusual conditions comprehensible. ”

“ While the alligator has a valve-like fold of skin in its throat that enables it to open its mouth and crush its prey under water, it is said that it must raise its head above water in order to swallow its food. A young alligator on land will usually throw back its head when trying to swallow a large piece of meat, so that it may be simply this motion that brings the head of the alligator above the surface of the water. ”

“ One afternoon the writer and a guide, while paddling along an old canal that was dug years ago into the Okefinokee Swamp, were preceded for perhaps half a mile by a large alligator that swam just fast enough to keep out of our reach until he came to the place where he wanted to turn off into the swamp.
Although so awkward on land, the alligator is said to be able to defend himself very effectively with his tail, which he sweeps from side to side with sufficient force, in the case of a large specimen, to knock a man off his feet. ”

“ The only difference between the two stages is that in the feeding the muscularis mucosa in the anterior region is much more strongly developed than in the hibernating stage; and in the former the nuclei of the epithelium are not arranged in two zones as in the latter.
The stomach has the usual layers, and has essentially the same structure in the three regions studied, except that the wall in the fundic region is much the thickest, due mainly to the great [191] thickness of the middle muscle layer. Only one kind of cell is found in the gastric glands. ”

“ These glands lie outside of the pelvis between the side walls of the cloaca and a large muscle that surrounds this part of the body. They have an oval form and open usually from their anterior end, sometimes just caudad to this, by a short, fairly wide, slit-like opening which has an anteroposterior direction. The walls of the glands are made up of three closely associated layers of connective tissue, the inner one being thrown into folds. Since these layers contain no muscle fibers the secretion of the gland is probably squeezed out by contraction of the circular muscles of the cloaca. ”

“ The epidermal skeleton of the alligator consists of oblong horny scales, arranged in transverse rows; the long axes of the scales are parallel to that of the body. On the tail, except along the mid-dorsal line, and on the ventral side of the trunk and head these scales are very regular in outline and arrangement; on the sides of the head and trunk and on the legs they are much smaller and less regularly arranged, while along the mid-dorsal line of the tail, especially in its posterior half, they are elevated into tall keels that give the tail a large surface for swimming. ”

“ During the earliest stages of development the embryo is very transparent; so that, as there is no fixed place upon the yolk at which it may be expected to occur, it is often very difficult to find. Owing to this transparency, to the extreme delicacy of the embryo, and to the character of the white, the removal of an early embryo from the egg of the alligator is a difficult operation and is accomplished only after some practice. ”

“ In the section under discussion the ectoderm (ec) is a very thin membrane on top of a considerable mass of yolk, while no entoderm can be distinguished. The amnion (a) completely surrounds [259] the embryo as an irregular membrane of some thickness in which no arrangement into layers can be seen. The epidermal ectoderm (ep) is composed of the usual loosely arranged cells, so that it is clearly distinguishable from the compactly arranged cells of the nervous layer (nl) , from which it is separated by only a line. ”

“ The hindbrain (hb) is wider than, but not so deep as, the forebrain; its walls are very thick laterally, but [285] are thin on the dorsal and ventral sides. The dorsal wall is reduced to a mere membrane, which, with the overlying ectoderm, has been pushed into the brain cavity, as is generally the case with such embryos. Close to the ventral wall of the hindbrain the notochord (nt) is seen. The character of the notochord has already begun to change; the cells are becoming rounded and vacuolated, with but few visible nuclei except around the periphery of the notochord. ”

“ The food of the adult alligator consists of fishes, birds, mammals, and possibly smaller individuals of its own species. The young eat small fish, frogs, insects, or worms.
If the animal be too large to swallow whole it is shaken and torn, the shaking being so vigorous that, according to Ditmars, the entrails of the prey may be thrown to a distance of twenty feet or more. Should two alligators seize the same prey at the same time they whirl about in opposite directions so violently that the prey is torn apart. ”

“ The postfrontal forms the antero-lateral boundary of the supratemporal fossa.
Articulating posteriorly and laterally with the parietal and the postfrontals, and forming the highest point of the skull, is the single frontal bone (24) , which, like the parietal, is paired in the embryo. It is a heavy bone whose dorsal surface is flattened posteriorly, deeply concave in the middle region, and drawn out into a long projection anteriorly. It forms part of the roof of the cranial cavity and articulates ventro-laterally with the alisphenoid and anteriorly with the prefrontals and nasals. ”

“ The crocodile is the nearest to the typical condition in the musculature of the tail. The lateral and ventral part of the tail musculature forms a broad mass that extends to the end of the tail; it lies immediately under the skin and springs from the caudal ribs (transverse processes—Gadow) and from the spinous processes of all the caudal vertebræ. The entire side musculature of the tail ends cephalad in several portions; the most ventral and medial of these bound the cloaca as an at least slightly developed, morphological sphincter ”

“ This membrane fastens the liver to the œsophagus.
The muscle of the right side is covered, on almost its entire inner surface (from its hinder end to the liver) by the belly-like skin, and is fairly closely united with it. The left muscle, on the other hand, is only covered by this skin from the hinder border of the stomach forwards; farther forward it lies immediately on the under and left side of the stomach and is united with it by loose connective-tissue. Outwardly both muscles are united by a thin layer of connective-tissue to the true abdominal muscles. ”

“ The Diaphragm (Diaphragmaticus, Zwerchfell, Bauchfellmuskel) . Closely inclosed between the skin and muscle of the abdomen, in the Crocodilia, [116] is a pair of muscles; they are, as a whole, thin muscles that are widely separated and extend in an anteroposterior direction. Each arises by two parts which, however, are united at the pelvis. One of these parts is small at its beginning, is fairly thick, and is attached by a short tendon, immediately over the pubis in front of the hip joint, to the ilium. ”

“ A strong, spindle-shaped muscle that arises, together with the ilio-fibularis, by a short tendon, from the side of the portio dorsalis of the ilium, and in the neighborhood of the bend of the knee is split into two tendons, of which the short one is inserted on the fibular side of the neck of the tibia, while the other, running along near the caput femoris of the gastrocnemius muscle, unites with the tendon of the caput tibiæ of the gastrocnemius muscle just beyond the ankle joint. ”

“ Near the posterior end of the figure the pelvic girdle (pl) is seen. The largest organ of the embryo, as seen in this section, is the heart, of which the ventricle (vn) seems to be closely surrounded, both in front and behind, by the auricles (au) . The liver (li) is the large, reticular mass back of the heart. Dorsal and anterior to the liver is the lung (lu) , now of considerable size and development. The enteron is cut in several places (oe, i) and its walls are beginning to show some differentiation, though this cannot be seen under the magnification here used. ”

“ Owing to the fact that the embryo may undergo considerable development before the egg is laid, and also to the unusual difficulty of removing the very young embryos, the earlier stages of development are very difficult to obtain.
The mesoderm seems to be derived chiefly by proliferation from the entoderm, in which way all of that anterior to the blastopore arises. Posterior to the blastopore the mesoderm is proliferated from the lower side of the ectoderm in the usual way. No distinction can be made between the mesoderm derived from the ectoderm and that derived from the entoderm. ”

“ In the lower jaw two masses of cartilage are seen, one near the symphysis (mk) and one near the wall of the trachea, doubtless the rudiment of the hyoid. The deep groove back of the Meckel’s cartilage (mk) marks the tip of the developing tongue, which here forms the thick mass on the floor of the mouth cavity. Dorsal to the pharynx a mass of cartilage (se) is developing in the sphenethmoid region. ”