Considering that the various stages in the formation of the posterior nares of the Amniota are so many repetitions of the adult states of lower forms, it may probably be assumed that the mode of formation of the posterior nares in Amphibia is secondary, as compared with that in the Amniota.
A diverticulum of the front part of the nasal cavity of the Anura is probably to be regarded as a rudimentary form of Jacobson’s organ.
Bibliography.
(394) G. Born. “Die Nasenhöhlen u. d. Thränennasengang d. amnioten Wirbelthiere.” Parts I. and II. Morphiologisches Jahrbuch, Bd. V., 1879.
(395) A. Kölliker. “Ueber die Jacobson’schen Organe des Menschen.” Festschrift f. Rienecker, 1877.
(396) A. M. Marshall. “Morphology of the Vertebrate Olfactory Organ.” Quart. Journ. of Micr. Science, Vol. XIX., 1879.
Sense organs of the lateral line.
Although I do not propose dealing with the general development of various sense organs of the skin, there is one set of organs, viz. that of the lateral line, which, both from its wide extension amongst the Ichthyopsida and from the similarity of some of its parts to certain organs found amongst the Chætopoda[197], has a great morphological importance.
The organs of the lateral line consist as a rule of canals, partly situated in the head, and partly in the trunk. These canals open at intervals on the surface, and their walls contain a series of nerve-endings. The branches of the canal in the head are innervated for the most part by the fifth pair, and those of the trunk by the nervus lateralis of the vagus nerve. There is typically but a single canal in the trunk, the openings and nerve-endings of which are segmentally arranged.
Two types of development of these organs have been found. One of these is characteristic of Teleostei; the other of Elasmobranchii.
In just hatched Teleostei, Schulze (No. [402]) found that instead of the normal canals there was present a series of sense bulbs, projecting freely on the surface and partly composed of cells with stiff hairs. In most cases each bulb is enclosed in a delicate tube open at its free extremity; while the bulbs correspond in number with the myotomes. In some Teleostei (Gobius, Esox, etc.) such sense organs persist through life; in most forms however each organ becomes covered by a pair of lobes of the adjacent tissue, one formed above and the other below it. The two lobes of each pair then unite and form a tube open at both ends. The linear series of tubes so formed is the commencement of the adult canal; while the primitive sense bulbs form the sensory organs of the tubes. The adjacent tubes partially unite into a continuous canal, but at their points of apposition pores are left, which place the canal in communication with the exterior.
Besides these parts, I have found that there is present in the just hatched Salmon a linear streak of modified epidermis on the level of the lateral nerve, and from the analogy of the process described below for Elasmobranchii it appears to me probable that these streaks play some part in the formation of the canal of the lateral line.