A first book in organic evolution

by D. Kerfoot Shute

Modified from Le Conte.

It is a very interesting and instructive fact that in the development of the human brain from the fertilized ovum these same stages, which are permanent conditions in the zoölogical (taxonomic) series, are passed through by it as transient stages.

One of the earliest conditions of the human brain is that in which it presents three swellings in a serial arrangement. They are known from behind, forwards as hindbrain, midbrain, and forebrain. For our purposes it is sufficiently accurate to say that the fœtal brain, in developing from this early condition to a later and higher condition, differentiates the hindbrain into the medulla ([Fig. 24], m) and the cerebellum (cb); the midbrain becomes the optic lobes (ol); and the forebrain differentiates into the thalami (th) and the cerebrum (cr). A little later the cerebrum buds forth the olfactory lobes (of), so that the human brain will consist of six fundamental segments,—one behind the other. This is the fish stage in the growth of the human brain. (Compare [Fig. 24] with [Fig. 19].)

Fig. 24.—Diagrammatic representation of the brain of a human fœtus of the third week. Representing the fish-phase in the ontogeny of the human brain. Side view. cr, cerebrum; th, thalamus; ol, optic lobes; cb, cerebellum; m, medulla. The olfactory lobes at this stage are very small and are not shown.

Fig. 25.—Dorsal view of the brain of a human fœtus of about seven weeks. Representing the reptilian phase in the ontogeny of the human brain. cr, cerebrum; th, thalami; ol, optic lobes; cb, cerebellum; m, medulla.

Fig. 26.—Side view of the brain of a human fœtus of about three months. Representing the marsupial phase in the ontogeny of the human brain. cr, cerebrum; ol, optic lobes; cb, cerebellum; m, medulla. The thalami are completely, and the optic lobes partially, covered by the greatly enlarged cerebrum.

As development proceeds the most conspicuous growth of the brain is observed in connection with the cerebrum and cerebellum. The cerebrum particularly grows relatively and actually larger and larger, but does not yet cover any portion of the optic lobes. This is the reptile stage, represented in [Fig. 25]. The cerebrum, continuing to grow, finally covers the front portion of the optic lobes. This is the marsupial stage, and is shown in Figs. [26] and [27]. Growing further, it soon covers a greater or less portion of the cerebellum. These are the prosimian (Lemur) and simian stages. Finally it grows so far backward as to completely cover the cerebellum, and so far forward as to project much beyond the olfactory lobes. This is the human stage ([Plate XII]).

Fig. 27.—Dorsal view of the brain (and spinal cord) of a human fœtus of about three months. Representing the marsupial phase of development. cr, cerebrum; ol, optic lobes; cb, cerebellum; m, medulla; bs, brachial enlargement of the spinal cord; ls, lumbar enlargement. The thalami are entirely covered and hidden from view by the cerebrum.