Fig. 20.—Curves of blastodermic layers and division of mesoderm in amniote embryo. (Kollmann.)
The folds of the splanchnopleure, indenting the yolk-sac, form a gutter directly connected with the yolk, the primitive intestinal groove or furrow, whose margins gradually approach each other (Fig. 20). In this way the primitive alimentary canal becomes separated from the yolk. At first this separation is ill-defined, and the channel of communication between the primitive intestine and the yolk is wide ([Figs. 13], [16], [17] and 19). The folding of the splanchnopleure completes, at an early period, the dorsal and lateral walls of the embryonic gut, but ventrally, toward the yolk, the tube is incomplete and widely open.
By union and coalescence of the splanchnopleural folds, proceeding from the caudal and cephalic ends towards the center, this primitive wide channel gradually becomes narrowed down, until the communication between the yolk-sac and the intestine is reduced to a canal, the vitello-intestinal or omphalo-mesenteric duct. The intestinal gutter is thus converted into a closed tube except at the point of implantation of the vitelline duct during the persistence of this structure. In the meanwhile the somatopleural folds forming the body-walls grow more and more together from the sides, approaching the vitello-intestinal duct. Finally touching each other they coalesce to form the ventral body wall, in the same manner as the splanch[n]opleural folds met and united to form the alimentary tube.
At the same time the vitello-intestinal duct and the remnant of the yolk-sac, to which it was attached (“umbilical vesicle”), normally become obliterated and disappear.
After the intestinal tube and the body cavity have thus become closed the embryo straightens out and the alimentary canal appears as a nearly straight cylindrical tube extending from the cephalic to the caudal end of the embryo. This primitive alimentary tube at first terminates at its cephalic extremity in a blind pouch, while at the caudal end in the early stages the intestine is connected with the nerve-tube by a channel called the neuro-enteric canal, forming in the earliest embryos a communication between the ectoderm lining the bottom of the medullary groove and the entoderm (Figs. 22 and [26]). In man this stage is encountered very early, in embryos of 2 mm. before the formation of either heart or provertebræ.
| Fig. 21.—Sagittal section of caudal extremity of cat embryo of 6 mm. (Tourneux.) |
| Fig. 22.—Caudal half of human blastoderm measuring 3 mm., with open medullary groove. Dorsal view. × 30. (After Spee.) |
At the point where the canal develops the primitive groove presents a thickened circumvallate spot, marking the beginning perforation of the medullary plate from the ectoderm to the entoderm. The canal exists only for a short period during the earliest stages of embryonal life. It becomes rapidly closed, the neural and intestinal tubes henceforth remaining permanently separated from each other.
The embryonal caudal end of the primitive alimentary canal is not the final adult termination of the tube. When the anal aperture is formed in a manner to be presently detailed, the opening is situated cephalad of the portion connected with the nerve-tube by the neuro-enteric canal. Hence this terminal portion of the early embryonic alimentary canal is called the “post-anal gut” (Fig. 21).
The post-anal gut and the neuro-enteric canal are better developed in the embryos of the lower than in those of the higher vertebrates. But in all vertebrates of the present day both of these structures undergo regressive changes and finally disappear altogether. They serve to recall conditions which existed in bygone ages, and, while they have a long and significant phylogenetic history, they have lost among living vertebrates all physiological importance.
After closure of the neuro-enteric canal and obliteration of the post-anal gut the alimentary tube ends, during a short period, both cephalad and caudad in a blind pouch. Very soon, however, the ectoderm becomes invaginated at both extremities and finally perforates into the lumen of the intestine, thus establishing the oral and anal communications with the exterior. The anal ectodermal invagination (proctodæum) (Fig. 21), is smaller than the oral (stomadæum) ([Fig. 27]), but the intestinal tube forms an extensive pouch in the anal region which descends to meet the ectodermal invagination of the proctodæum. The details of the embryonic processes leading to the final establishment of the adult condition are of great interest on account of the pathological importance of abnormal or arrested development in these parts. Failure of the caudal intestinal pouch to establish a communication with the anal invagination, or failure of development in either anal invagination or intestinal pouch, leads to the condition known as atresia ani or imperforate anus, of which there are several varieties.