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Show 238 ON THE STTIUOTU~E OF THE SKU!Jh proceed ; and, further forwards, the hemispheres of the ce: ebrum. Anteriorly, both trabeculro reached as far as the unteno~· end of the head and here bent slightly upwards, so that they proJected a little in;o the frontal wall of the head, th eir ends lying in front of the eerebrum. Almost at the end of eaeh horn, however, I saw a small process, its immediate prolongation, pass outwards and form, as it were, the nucleus for a small lateral projection of the nasal process of the. frontal wal.l. The middle trabecula grows, with the brain, further and further into the cranial cavity; and as the dura mater begins to be now distinguishable, it becomes more readily obvious than before, that the middle trabecula raises up a transverse fold of it, which traverses the cranial cavity transversely.* The fold itself passes laterally into the cranial wall; it is highest in the middle, where it encloses the median traboeula, and becomes lower externally, where it forms, as it were, a short ala proceeding from the trabecula. With increasing elongation, the trabecula becomes broader and broader towards its free end, and, for a short time, its thickness increases. After this, however, it gradually becomes thinner, without any change in its tissue, till, at the end of the second period, it is only a thin lamella, and after a short time (in the third period) entirely disappears. In Mammals, Birds, and Lizards, that is, in those animals in general in which the middle cerebral vesicle is very strongly bent up and forms a protuberance, while the base of the brain exhibits a deep fold between the infundibulmn and the posterior cerebral vesicle, a similar part to this median trabecula of the skull is found. · In these animals, also, at a certain very early period of embryonic life, it elevates a fold of the dura mater which passes from one future petrous bone to the other, and after a certain time projects strongly into the cranial eavity. Smnewhat later, however, it diminishes in height and thickness, as I have especially observed in embryos of the Pig and Fowl, until at last it disappears entirely in these higher animals also, the two layers of the fold which it had raised up coming into contact. When *What Rathke terms tho "middle trabecula," appears to be only very indis· tinetly developed in Fishes nnd Amphibia. :.. THE Sl{ULJ.S OF REPTILIA AND AVES. 239 this has happened, the fold diminishes in height and eventually vanishes, almost completely. The t':o lateral trabeculre,. whieh in the Snake help to fonn th~ .ant~nor half of th~ bas1s of the skull, attain a greater solidity In the second penod, acquire a greater distinctness from the surrounding parts, and assume a more determinate form becoming, in fact, filiform; so that the further forward th~ thinner they appear. They increase only very little in thiekness, but far more in length, during the growth of the head. Altoget~er ~nteriorly, they coalesce with one another, forming a part winch hes between the two olfactory organs and constitutes a ~eptum .. As soon as these organs increase markedly in size, this part Is moderately elongated and thickened, without however, becoming eo dense as the hinder, longer part of the trabeculre. The prolongations into the lateral projections of the nasal. processes, which now. proceed from the coalesced part in questwn, also become but httle denser in texture for the present though they elongate considerably. ' The lateTal parts and the upper wall of the cranium with the except~on of th~ auditory capsules, or of the subse~uent bony labynnth, remmn merely membranous up to the end of the second period; consisting, in fact, only of the cutaneous ?overing, the dura mater, and a little interposed blastema, which Is hardly perceptible in the upper part, but increases in the lateral walls, towards the base of the skull. The notochord reaches, in very young embryos of the Sn.ake? to between the auditory capsules; and further than this pmnt It can be traced neither in the Snake nor in other Verteb~ ata, at a~y period of life, as manifold investigations, conducted With espemal reference to this point, have convinced me. At. the beginning of the third period, the basal plate chondnfies, at first leaving the space beneath the middle of the cereb~llu~ ~embranous; but this also eventually chondrifies, an~ Is distinguished from the rest of the skull only by its thinness. Lateral processes grow out from the basal cartilage just in front of the occipital foramen, and eventually almost meet above. They are the ex-occipitals. |