The primitive condition of the embryonal mammalian alimentary tract, after differentiation of the large intestine is well illustrated by some of the lower vertebrates in which development never proceeds beyond this stage. [Fig. 112] shows the entire alimentary canal of a teleost fish, the conger eel (Echelus conger) isolated.
The preparation forms a good illustration of the embryonal stage of the higher vertebrates in which development has not proceeded beyond the formation of the simple umbilical loop, about corresponding to the schematic [Fig. 98]. The stomach is differentiated both by its caliber and by the formation of a pyloric ring valve.
The midgut forms a simple loop with a descending and ascending limb closely bound together by mesenteric attachment. Different from the course of development followed in the human embryo is the situation of the ileo-colic junction. The same appears in the terminal straight segment of the canal—corresponding to the human descending colon—while in the human embryo the differentiation of small and large intestine takes place in the course of the ascending limb of the loop. This condition depends upon the relatively much shorter extent of the teleost endgut compared with the human large intestine. Other examples are afforded by the alimentary tract of some of the Amphibia and Reptilia. [Fig. 105] shows the alimentary canal of Rana catesbiana, the common bull frog. The stomach, fairly well differentiated, is succeeded by the small intestine of considerable length and uniform caliber. The proximal portion of the small intestine is characterized as duodenum by its connection with liver and pancreas. In the remaining portion of the intestinal canal it is not difficult to recognize the elements of the umbilical loop of the higher mammalian embryo. The larger mass of the jejuno-ileal coils is developed from the descending limb of the loop; a smaller number of convolutions belong to the returning or ascending limb, which also includes the ileo-colic junction. The very short large intestine of the frog passes straight down to enter the cloaca. Another example, in which the early embryonal stages of the higher mammalia are illustrated by the permanent structure of one of the lower vertebrates, is given in [Fig. 106], which shows the alimentary tract of a chelonian, Pseudemys elegans, the pond turtle. The bilobed liver fits over the well-differentiated stomach in the manner of a saddle. The stomach itself, as in chelonians generally, has a markedly transverse position and passes under cover of the right lobe of the liver into the duodenum. The coils of small intestine form a prominent mass, which, however, when unravelled as shown in the figure, permits us to recognize its identity with the mammalian embryonic umbilical loop. The well-marked ileo-colic junction is situated at the termination of the returning limb of the loop, close to the beginning of the descending limb. This close approximation of the duodenum and colon (duodeno-colic isthmus) forms one of the most important factors in the further development of the mammalian intestinal canal and will again be referred to below.
Fig. 107.—Abdominal viscera of Tamandua bivittata, the little ant-eater, seen from the left, with the intestines turned to the right. (From a fresh dissection.)
Fig. 108.—The same view, from another specimen. Figures 107 and 108 should be studied and compared together, as each supplements the other.
Fig. 109.—Abdominal viscera of Tamandua bivittata, the little ant-eater, seen from the right, with the intestines turned to the left. (From a fresh dissection.)