[508] This statement of Agassiz does not correspond with the dates on the specimens sent to us—a fact no doubt due to the hatching not taking place at the same time for all the larvæ.
Brain.
I. Anatomy.
The brain of Lepidosteus has been figured by Busch (whose figure has been copied by Miklucho-Maclay, and apparently by Huxley), by Owen and by Wilder (No. 15). The figure of the latter author, representing a longitudinal section through the brain, is the most satisfactory, the other figures being in many respects inaccurate; but even Wilder's figure and description, though taken from the fresh object, appear to us in some respects inadequate. He offers, moreover, fresh interpretations of certain parts of the brain which we shall discuss in the sequel.
We have examined two brains which, though extremely soft, were, nevertheless, sufficiently well preserved to enable us to study the external form. We have, moreover, made a complete series of transverse sections through one of the brains, and our sections, though utterly valueless from a histological point of view, have thrown some light on the topographical anatomy of the brain.
Plate 38, figs. 47A, B, and C, represent three views of the brain, viz.: from the side, from above, and from below. We will follow in our description the usual division of the brain into fore-brain, mid-brain, and hind-brain.
The fore-brain consists of an anterior portion forming the cerebrum, and a posterior portion constituting the thalamencephalon.
The cerebrum at first sight appears to be composed of (a) a pair of posterior and somewhat dorsal lobes, forming what have usually been regarded as the true cerebral hemispheres, but called by Wilder the prothalami, and (b) a pair of anterior and ventral lobes, usually regarded as the olfactory lobes, from which the olfactory nerves spring. Mainly from a comparison with our embryonic brains described in the sequel, we are inclined to think that the usual interpretations are not wholly correct, but that the true olfactory lobes are to be sought for in small enlargements (Plate 38. figs. 47A, B, and C, olf.) at the front end of the brain[509] from which the olfactory nerves spring. The cerebrum proper would then consist of a pair of anterior and ventral lobes (ce.), and of a pair of posterior lobes (ce´.), both pairs uniting to form a basal portion behind.
The two pairs of lobes probably correspond with the two parts of the cerebrum of the Frog, the anterior of which, like that of Lepidosteus, was held to be the olfactory lobe, till Götte's researches shewed that this view was not tenable.
The anterior lobes of the cerebrum have a conical form, tapering anteriorly, and are completely separated from each other. The posterior lobes, as is best shewn in side views, have a semicircular form. Viewed from above they appear as rounded prominences, and their dorsal surface is marked by two conspicuous furrows (Plate 38, fig. 47B, ce´.), which have been noticed by Wilder, and are similar to those present in many Teleostei. Their front ends overhang the base of the anterior cerebral lobes. The basal portion of the cerebrum is an undivided lobe, the anterior wall of which forms the lamina terminalis.