Hence the appendices were regarded as homologous with the pancreas of the higher forms. Recent observations have shown that this view is not strictly and entirely correct, while at the same time it merits consideration in several respects.
It is true that the pancreas in certain teleosts is now known to be present although concealed from observation in the liver or scattered in the form of small lobules between the layers of the mesentery (cf. p. 117), and that in a number of fish, such as _Salmo salar_, _Clupea harengus_, _Accipenser sturio_, both the appendices and the pancreas are encountered. Consequently these structures are not identical or even completely homologous, since they occur side by side in the same form.
On the other hand Krukenberg has demonstrated that the appendices pyloricæ may function physiologically as a pancreas by yielding a secretion which corresponds to the pancreatic juice in its digestive action. In the majority of forms, however, they apparently merely increase the intestinal absorbing surface, secreting only mucus.
These structures are nevertheless very interesting and instructive since they furnish a perfect gross morphological illustration of the embryonal stages just considered in connection with the development of the mammalian pancreas. In the adult ganoid or teleost these blind diverticula or pouches, varying greatly in shape, number and size, protrude from the intestine immediately beyond the pylorus, usually in close connection with the duodenal entrance of the bile-duct. Two or more of these pouches may unite to form a common duct or canal opening into the intestine.
These forms, therefore, offer direct and valuable morphological illustration of the manner in which the pancreas of the higher vertebrates develops, _i. e._, as a set of hollow outgrowths or diverticula from the hypoblast of the primitive enteric tube. We can establish a consecutive series, beginning with forms in which only one or two diverticula are found, and extending to types in which the number of the little cylindrical pouches reaches nearly two hundred and in which they are bound together by connective tissue and blood vessels so as to closely resemble the structure of a glandular pancreas. This is one of the most striking instances in which the minute embryological stages of the higher types are directly illustrated by the permanent adult conditions found in the lower vertebrates. [The same statement, as we will see, holds good in reference to the development of the _liver_.]