202 SOMMAEY OF CURRENT RESEARCHES RELATING TO 



beginning to be developed ; it presents a stage in formation which has 

 never yet been recorded. The distal layer of the optic cup now 

 exhibits the first indications of striation, and an inner thinner layer 

 of cells with more rounded nuclei may be distinguished from an outer 

 thicker one in which the nuclei are more elongated ; between them 

 lies a clearer zone, poor in cells. The primitive optic nerve is still 

 hollow, though signs of closure are very apparent; it consists 

 partly of the primitive elements of the medullary plate, and partly 

 of very fine longitudinal fibrils which are superficial in position, and 

 extend through the whole length of the nerve. The pigment-layer 

 was intensely brown, and in its thinnest part consisted of two layers 

 of cells. The lens presented the condition in which the lens-fibres 

 had begun to be formed from the cells of the hinder wall of the 

 primitive vesicular rudiment ; the capsule was very distinct, and of 

 the same thickness throughout. A distinct cornea was already present, 

 and, in nature, clearly stands in direct contact with the lens-capsule. 

 The mesodermal tissue around the eye was thickened, but was not yet 

 marked off externally. 



The oldest embryo had a spherical lens, the anterior aqueous 

 chamber, and eyelids; the optic nerve exhibited no sign of any cavity, 

 and consisted only of the network of stellate cells derived from the 

 primitive nerve, and very fine non-nucleated optic fibrils ; the lens 

 had lost all cavity within, the retina consisted anteriorly of elongated 

 cells, and here and there zones poor in or free of nuclei could be 

 made out. The uvea and sclerotic were not yet distinctly differentiated, 

 but formed only a somewhat thick tissue around the eye. The cornea 

 was remarkably delicate. There were no signs of lachrymal glands, 

 but the ducts and canaliculi were well developed. 



In the youngest embryo the uppermost end of the olfactory pit 

 formed already a blind sac, the true olfactory blind-sac, which becomes 

 later on the uppermost part of the olfactory region ; in the oldest, the 

 olfactory pits were connected with the primitive buccal cavity by 

 ducts — nasal ducts, the labial and mandibular clefts were seen to be 

 closed, but the palatal cleft was still open. A study of this specimen 

 shows that the network of stellate cells of the olfactory lobe is con- 

 verted, either partially or completely, into a nucleated network of fine 

 bundles of the finest olfactory fibrils ; that the network of cells grow 

 out from the olfactory lobe either before or simultaneously with its 

 conversion into fibrils ; it gives rise to buds of cells which project 

 into the mucous membrane of the nose, while behind these buds it 

 continues to be converted into a fibrillar network. The nucleated 

 fibrillar bundles found in the embryo are the predecessors of the 

 nucleated pale olfactory fibres of the adult. If this description is 

 exact it will follow that the fibres of the olfactory nerve are compar- 

 able to the axis- cylinders of other nerves, and their nuclei to the 

 nuclei of nerve-cells. For man, as for other forms, embryology shows 

 that the olfactory lobe is a part of the brain, that the point of origin 

 of the nerves is to be found in the primitive olfactory lobes, that the 

 nerves grow out from the lobes, or from parts derived therefrom, 

 and that the olfactory tract and radices are secondary commissural 



