. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. THE CEREBRAL CONVOLUTIONS 403 the fissure may be termed the hippocampal fissure. The portion of the pallium which intervenes between this fissure and the chorioidal forms what is known as the dentate gyrus. Toward the end of the third or the beginning of the fourth month two prolongations arise from the fissure just where it turns to be continued into the temporal lobe, and these, extending posteriorly, give rise to the parieto-occipital and calcarine fissures. Like the hippocampal, these fis
. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. THE CEREBRAL CONVOLUTIONS 403 the fissure may be termed the hippocampal fissure. The portion of the pallium which intervenes between this fissure and the chorioidal forms what is known as the dentate gyrus. Toward the end of the third or the beginning of the fourth month two prolongations arise from the fissure just where it turns to be continued into the temporal lobe, and these, extending posteriorly, give rise to the parieto-occipital and calcarine fissures. Like the hippocampal, these fissures produce elevations upon the inner surface of the pallium, that formed by the parieto-occipital early disappearing, while that pro- duced by the calcarine persists to form the calcar {hippocam- pus minor) of adult anatomy. The three fissures just described, together with the chorioidal and the lateral cerebral fossa, are all formed by the beginning of the fourth month and all the fissures affect the entire thickness of the wall of the hemisphere, and hence have been termed the primary or total fissures. Until the beginning of the fifth month they are the only fissures present, but at that time secondary fissures, which, with one exception, are merely furrows of the sur- face of the pallium, make their appearance and continue to form until birth and possibly later. Before considering these, however, certain changes which occur in the neighborhood of the lateral cerebral fossa may be described. The fossa is at first a triangular depression situated above the temporal lobe on the surface of the hemisphere. During the fourth month it deepens considerably, so that its upper and lower margins become more pronounced and form projecting folds, and, during the fifth month, these two folds approach one another and eventually. Fig. 242.—Brain of an Embryo of the Fourth Month. c, Cerebellum; p, pons; s, lateral cerebral Please note that these images are extracted from scanne
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