. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. THE DIENCEPHALON 397. from the roof-plate of the first brain vesicle, further forward, in the region which becomes the median portion of the telencephalon. This paraphysis as it has been called, has been observed in the lower vertebrates and in the Marsupials (Selenka), but up to the present has not been found in other groups of the Mammalia. It seems to be comparable to a chorioid plexus which is evaginated from the brain surface instead of being invaginated as is usually the case. There is
. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. THE DIENCEPHALON 397. from the roof-plate of the first brain vesicle, further forward, in the region which becomes the median portion of the telencephalon. This paraphysis as it has been called, has been observed in the lower vertebrates and in the Marsupials (Selenka), but up to the present has not been found in other groups of the Mammalia. It seems to be comparable to a chorioid plexus which is evaginated from the brain surface instead of being invaginated as is usually the case. There is no evi- dence that a paraphysis is developed in the human brain. The portion of the roof-plate which lies in front of the epiphysis represents the velum interpositum of the adult brain, and it forms at first a distinct ridge (Fig. 239, rp). At an early stage, however, it be- comes reduced to a thin membrane upon the surface of which blood- vessels, developing in the surround- ing mesenchyme, arrange them- selves at about the third month in two longitudinal plexuses, which, with the subjacent portions of the velum, become invaginated into the „ „, 0 riG. 239.—Dorsal View of the cavity of the third ventricle to form Brain, the Roof of the Lateral its chorioid Mexu* Ventricles being Removed, of an us cnomoia plexus. Embryo of mm. The dorsal zones thicken in b, Superior brachiuui; eg, lateral their more dorsal and anterior Seniculate. body; CP> chorioid plexus; tneir more aorsai ana anterior cqa> anterior corpu3 quadrigeminum; portions to form massive Structures, h> hippocampus; hf, hippocampal fis- , 7 7 . , sure; ot, thalamus; p, pineal body; rp, the thatami [rigs. 233, V2, and roof-plate.—(Aw.) 239, ot), which, encroaching upon the cavity of the ventricle, transform it into a narrow slit-like space, so narrow, indeed, that at about the fifth month the inner surfaces of the two thalami come in contact in the median line, forming what is known as the intermed
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