. Text-book de l'embryologie. L'embryologie. Glande pinéale OEGAN 99 assume une forme lenticulaire, ses cellules devenant beaucoup tout en restant allongé dans une seule couche. Cet épaississement lenticulaire devient parfois perdu au cours du développement, un fait qui peut être considéré comme constituant un élément de preuve en faveur de l'avis que l'œil, à l'heure actuelle, est dans une phase régressive de l'évolution. Les parties de la paroi vésiculaire qui ne participent pas à la formation de l'objectif l'objet d'histogenetic change en tissus de la rétine. Les cellules subissent une différenciation dans les deux sens. L'un ensemble devenu cellules pigmentaires-

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. Text-book of embryology. Embryology. PINEAL OEGAN 99 assumes a lenticular form, its cells becoming much elongated though remaining in a single layer. This lenticular thickening occasionally becomes lost during development—a fact which may be taken as forming a piece of evidence in favour of the view that the eye, at the present time, is in a retrogressive phase of evolution. Those parts of the vesicle wall which do not take part in the formation of the lens undergo histogenetic changes into retinal tissue. The cells undergo differentiation in two directions. The one set become pigment cells—tall columnar cells which traverse the whole thickness of the retina and have their nuclei towards the basal or outer end and which develop dark melanin granules in their proto- plasm. Interspersed with these are the percipient cells, shorter in form, their basal ends not reaching the outer surface, and carrying at their inner ends cilium-like structures which project into the cavity of the vesicle. The idea that these projec- tions correspond physiologically to rods appears to be negatived by the fact that they occur also on the inner ends of the cells forming the lens. At their basal ends these cells are continued into nerve-fibres, which form a distinct layer internal to the nuclei of the pigment cells and are eventually continuous physiologi- cally with the fibres of the pineal nerve. Scattered amongst, and in the neighbourhood of, this fibrous layer ganglion -cells are present: they are about the first definite elements to become recognizable during the histogenesis of the retina and appear first close to the point of attach- ment of the optic nerve. The cavity of the pineal eye is kept distended by a clear substance, the vitreous body, and this is colonized by a certain number of cells (see Fig. 55, 0) which are most probably to be regarded as immi- grant mesenchyme cells. In Sphenodon, the sole survivor of the. only other existing group of Eeptiles in which a pineal

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