Fig. 51.—Sub-fish Stage. th, thalamus; ol, optic lobe; m, medulla.
Fig. 52.—Fish-stage. of, olfactive lobe; cr, cerebrum; th, thalamus; ol, optic lobe; cb, cerebellum; m, medulla.
Fig. 53.—Reptile-stage.
Now, it is a most remarkable fact that substantially these same stages, which are permanent conditions in the taxonomic series, are passed through as transient stages in the embryonic development of the human brain, and in the order given above. The very early condition of the human brain is represented in [Fig. 51]. It is evidently nothing more than the intercranial continuation of the spinal cord, enlarged a little into three swellings or ganglia. These are the early representatives of the medulla, the optic lobes, and the thalamus; which last may be regarded as the basal and most fundamental part of the cerebrum. This stage may be regarded as lower than that of the ordinary fish. I have called it, therefore, the sub-fish stage. The cerebellum is a subsequent outgrowth from the medulla, as is the cerebrum and olfactive lobes from the thalamus. [Fig. 52] may be said, therefore, to represent fairly the fish-stage. Henceforward the principal growth is in the cerebrum and cerebellum, both of which are subsequent outgrowths of the original simple ganglia, the medulla, and the thalamus. The cerebrum especially increases steadily in relative size, first becoming larger than but not covering the optic lobes ([Fig. 53]). This represents the reptilian stage. Next, by further growth, it covers partly the optic lobes ([Fig. 54]). This may be called the bird-stage. Then it covers wholly the optic lobes, and encroaches on the cerebellum behind and olfactive lobes in front ([Fig. 55]). This is the mammalian stage. Finally, it covers and overhangs all, and thus assumes the human stage ([Fig. 56]).
Fig. 54.—Bird-stage. of, olfactive lobe; cr, cerebrum; th, thalamus; ol, optic lobe; cb, cerebellum; m, medulla.
