IV. VERTEBRATA. 477 



This peculiar and functionally purposeless inversion of the vertebrate retina 

 is explained by the development of the eye. This can be divided, according to 

 origin, into two parts, a cerebral (optic nerve, retina, tapetum) and a peripheral 

 (all other parts). As the eye in tunicates and Amphioxus is permanently a part 

 of the brain, so is the retina of vertebrates genetically, coming from the first 

 cerebral vesicle. An outgrowth occurs on either side (fig. 531, B) of the 'twi.xt 

 brain and becomes expanded distally to an optic vesicle which is connected with 

 the brain by an optic stalk. The vesicle extends out to the periphery and, coin- 

 cidentally with the development of the lens, is folded into a double-walled optic 

 cup with outer or lapetal, inner or retinal layers. If the position of the epithelial 

 cells be followed (fig. 531), it will be seen that the peripheral ends rest upon the 

 tapetum, and when these ends develop the rhabdomes, these must grow into the 

 tapetal layer. 



Fig. 531. — Diagram showing the inversion of layers in the formation of the retina 

 (orig.). The nuclei are placed in the (morphologically) deeper ends of Ihe cells. In .4 

 the brain (6) has been closed in; in B the optic vesicle (v) has reached the lens (/) and 

 on the right is being converted into the double-walled optic cup with, as shown in C, 

 an outer tapetal (e) and an inner retinal layer (;•). 



In contrast to the retina, the lens develops as an invagination from the epi- 

 thelium of the body (/) ; sclera, cornea and vitreous body from connective tissue. 

 Thus the important part of the eye arises from the brain and is later provided with 

 accessory apparatus which arise from peripheral parts The invertebrate eye, 

 on the other hand, with all its parts arises from the skin. 



The vertebrate eye is furnished with secondary structures: with muscles 

 which move it, with lids which protect the cornea from injury and drying. The 

 lids are dermal folds which extend over the eyeball from above and below. The 

 lids enclose a conjunctival sac filled with lacrimal fluid, and bounded by the 

 epithelium (conjunctiva) which covers the cornea. To these a third lid, the 

 nictitating membrane, may be added. It arises as a conjunctival fold from the 

 inner angle of the eye, and can extend over the cornea beneath the upper and 

 lower lids. A special lacrimal gland, which occurs at the outer angle of the eye, 

 provides the fluid to moisten the cornea, while a second or Harder s gland occurs 

 at the inner angle when a nictitating membrane is present. Both are lacking 

 in the fishes; there is a single gland in the Amphibia. 



The ear, at the level of the medulla oblongata, has one point in com- 

 mon with the invertebrate otocyst — it arises as an ectodermal pit which 

 is usually completely cut off from its parent layer, and only in elasmo- 

 branchs (fig. 532, II) remains connected with the e.xterior by a tube, the 

 elsewhere closed endolymphatic duct. In the cyclostomes (fig. 532, 7) it 

 consists of a single vesicle with a single patch of sensory cells, the macula 

 acustica; from the fishes upwards the vesicle becomes constricted into an 



