717. 3. Next, the vesicle, instead of being rounded, is elongated into a peduncle or neck, not unlike the neck of a bottle (fig. [CLXXXII]. 1). This pedunculated vesicle is called a follicle.
718. 4. Then, the follicle is somewhat elongated, without neck and without terminal expansion (fig. [CLXXXVI]. 1); and this is called a cæcum or pouch.
719. 5. And, lastly, the cæcum itself is elongated; so that instead of presenting the appearance of a pouch, it rather resembles a tube (fig. [CLXXXV]. 1), and is accordingly named tubulum.
720. In the construction of the secreting apparatus, membrane, then, may be said to be disposed into four elementary forms constituting cryptæ or vesicles, follicles, cæca and tubuli. Membrane, disposed into these elementary forms, constitutes the simple bodies by the accumulation and the varied arrangement of which the compound organs are composed. There is no other known element which enters into the composition of the most complex secreting organ.
721. One of these elementary bodies may exist as a simple organ, or many may be collected into a mass to form a compound organ. When single they are called solitary: when collected into a mass, aggregated. Each elementary body has a mode of aggregation peculiar to itself. Vesicles aggregate by clustering together (fig. [CXXXVIII].), and adhering as if by a common stem (fig. [CXXXVIII].); follicles by uniting at their orifices (fig. [CLXXXIII].), and forming masses which are disposed either in a linear direction (fig. [CLXXXIII].) or in fasciculi (fig. [CLXXXIV].); cæca by forming bundles, parallel or branched (fig. [CLXXXVI].); and tubuli by forming masses straight (fig. [CLXXXV].), tortuous or convoluted (figs. [CLXXXV]. and [CLXXXIX].).
Aggregated follicles disposed in a linear direction, here represented of their natural size, as seen near the mouth in the goose.
