. Contributions to embryology. Embryology. 18 CYCLOPIA I\ THE HUMAN EMBRYO. anterior end of the medullary plate. According to Lewis, certain groups of cells in the medullary plate are predetermined to form the tapetum, the retina, and the optic stem. Lewis's theorj^ has been objected to by Bell, but it has been amply confirmed by Spemann. At any rate, the arrangement of the structures in our cj'clopean embryo indicates that the optic stems have been cut out and that the primordia of the retina and tapetum of the two sides have united, blending abso- lutely with each other across the midventral
. Contributions to embryology. Embryology. 18 CYCLOPIA I\ THE HUMAN EMBRYO. anterior end of the medullary plate. According to Lewis, certain groups of cells in the medullary plate are predetermined to form the tapetum, the retina, and the optic stem. Lewis's theorj^ has been objected to by Bell, but it has been amply confirmed by Spemann. At any rate, the arrangement of the structures in our cj'clopean embryo indicates that the optic stems have been cut out and that the primordia of the retina and tapetum of the two sides have united, blending abso- lutely with each other across the midventral line. The tissues of the midbrain and most of those of the interbrain appear to be normal, but our knowledge of the normal brain at this period of developrnent is so scanty that it is dangerous to make anj^ defi- nite statement. Single groups of cells may be wanting or may be blending without our noticing the change. Such a blending is clear only when it involves a sharply circumscribed structure like the eye. However, the tissues of the hypothalamus seem to be disarranged (plate 3, fig. 2), and those of the single imited cerebral vesicle (plate 3, fig. 5) are certainly dissociated. In the cerebral vesicle the cells form a uniform layer, which is not beautifully stratified, as is the case in normal develojmient. Over the most anterior part of the brain (plate 3, fig. 5), cross- ing the midline, is a crescent- shaped cap covering the outside of the brain and reaching back to the hyiK)thalamus just below the i)oint of attachment of the common optic stem (plate 3, fig. 3). This cap is composed of i)ale cv\U of uniform size, undoubtedly belonging to the neural tube. It is located on the j^art of the brain which gives rise to the olfactory lobes in normal development; and it may represent these lobes in a degenerated form. It was necessary to make a model (enlarged 100 diameters) of the eye region in order to study carefully the anatomy of the structures of the orbit. In this model
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Keywords: ., bookauthorcarn, bookpublisherwashingtondc, booksubjectembryology
