DEVELOPMENT OF THE EYE. 965 



tion, and thus the future corpus callomm is formed with the fornix, 

 composed of longitudinal fibres, beneath it, and the septum lucidum, 

 inclosing the cavity of the fifth ventricle, between them. From the 

 under-surface of the anterior part or frontal lobe of each hemisphere, 

 a hollow process extends forward, forming the future olfactory lobes, 

 the central cavities in which, in some animals, remain in communica- 

 tion with the ventricles of the hemispheres, but in others are obliter- 

 ated. From the hinder and lateral part of the anterior cerebral vesicle, 

 the primary optic or ocular vesicle, or rudimentary eye, is developed, 

 forming connections with the optic thalamus and corpus quadrigemi- 

 num. From both these latter parts, which are then hollow, two tubu- 

 lar processes of nervous substance extend forward to the optic vesicles, 

 and are ultimately developed into the optic tracts and optic nerves. 

 Farther back, on the sides of the future medulla oblongata, are the 

 primary auditory sacs or vesicles, which are not developed, like the 

 ocular vesicles, as outgrowths of the cerebral vesicles, but commence 

 on the surface of the embryo, as will be immediately described. 



The membranes of the brain and cord are formed between the ner- 

 vous centres and the walls of the cranial and spinal cavities. The pia 

 mater is first recognizable. 



The sympathetic nervous system is said to be developed from the 

 middle germinal layer. 



The Organs of the Senses. 



The Nose. As the olfactory lobes become consolidated, the nasal 

 cavities, with the olfactory lining membrane, are developed, as inversions 

 of the integument of the face, in the so-called primary olfactory groove. 

 This, remaining open, becomes subdivided, to form the two nasal pas- 

 sages or fossae. At one time, these fossae are closed at the bottom, a 

 condition which is permanent in Fishes; afterwards they communi- 

 cate, in front of the palate, with the mouth, as in certain Amphibia; 

 finally, they open only into the pharynx, as in Reptiles, Birds, and 

 Mammals. 



The Eye. The primary ocular or optic vesicles commence, as al- 

 ready mentioned, as flask-shaped outgrowths of the first cerebral vesi- 

 cle, with which they soon appear connected by a hollow stalk, the 

 future optic tract and nerve. The interior of each optic vesicle quickly 

 becomes lined with nervous substance. At the same time, the sur- 

 face of the common integument covering the vesicle presents an 

 inversion of the epidermic layer, which, becoming constricted at its 

 orifice, closes and forms a sac; this is ultimately converted into the 

 capsule of the lens, within which the lens-fibres are gradually developed, 

 from radiating, epidermoid, nucleated cells. This growth, with other 

 deeper tissues, pushes, as it were, the anterior and lower part of the 

 nervous layer of the primary optic vesicle upwards and inwards, 

 against the posterior and upper part, giving rise to a cup-shaped ner- 

 vous expansion, open below, named the secondary optic vesicle, within 

 which the vitreous humor is developed, this being also, like, the lens, 

 an integumentary structure. The sides of this secondary vesicle, con- 



