In the Cyclostomata ([Fig. 43]) the intestinal canal passes through the body in a perfectly straight line and the three segments (mid-, fore- and hindgut) are not clearly differentiated.

In the Ammocœtes the foregut begins behind the wide branchial basket, dorsad of the heart, with a narrow entrance, which is succeeded by a dilated segment. The entrance of the hepatic duct separates fore- and midgut.

In Amphioxus the branchial pouch passes with a slight constriction directly into the gut which extends through the body-cavity in a straight line.

The narrow segment is usually regarded as the “œsophagus.” This is followed by a slightly dilated segment, the “stomach,” into which a blind pouch enters. This cæcal pouch is usually considered as a hepatic diverticulum ([Fig. 49]).

But even in these rudimentary forms the point where the liver develops from the entodermal intestinal tube marks the separation of fore- and midgut. The stomach, when it develops, is situated cephalad of the entrance of the hepatic duct into the intestine. The section cephalad of the duct opening may be very short, and the food digested further on in the intestinal tube. Consequently a function which in these lower vertebrates is assigned to the midgut becomes transferred in the higher forms to a specialized segment of the foregut, situated cephalad of the hepato-enteric duct. This segment is the . . .

Fig. 48.—Alimentary canal of Belone, pick­erel. (Nuhn.)   Fig. 49.—Amphioxus, dissected from the ventral side. The relatively enormous pharynx occupies more than half the length of the body. The walls are separated by the gill-clefts, and the parallel gill-bars abut at the midventral line on the endostyle. (Willey, after Rathke.)  Fig. 50.—Necturus maculatus, mud-puppy. Ali­men­tary ca­nal and appendages. (Columbia University Museum, No. 1454.)  Fig. 51.—Ali­men­tary ca­nal of Proteus angui­neus. (Nuhn.)

STOMACH.

The distribution of the vagus nerve finds its explanation in this derivation of the stomach. The primitive foregut is formed by the passage between the branchial cavity and the midgut, and is within the area supplied by the vagus. Hence when the stomach develops from the foregut, as a specialized segment of the same, it is supplied by vagus branches. The vertebrate stomach varies greatly in size and shape.

The type-form is presented by a longitudinal spindle-shaped dilatation of the foregut, which retains its fœtal vertical position in the long axis of the body. An example of this form, which is encountered among fishes and amphibia, is presented by the alimentary tube of Proteus anguineus and Necturus maculatus (Figs. 50 and 51). Since this condition is common to all vertebrates in the earliest fœtal period it can be designated as the fœtal or primitive stomach form. All others appear as secondary derivatives from this typical early condition.

The influences which bring about such derivations and modifications may be enumerated as follows: