498 STRUCTURE OF VERTEBRATA. 



one next the cavity of the cup becomes the retina, while the 

 outer forms the pigmented epithelium and the muscles of 

 the iris. Meanwhile, surrounding mesoblast has insinuated 

 itself past the lens into the cavity of the optic cup, there 

 forming the vitreous humour, while externally the mesoblast 

 also forms the vascular choroid, the firm often cartilaginous 

 sclerotic, the inner layer of the cornea, etc. Along the 

 thinned stalk of the optic cup the optic nerve is developed. 

 Its protective sheath is continuous with the sclerotic of the 

 eye and the dura mater of the brain. As the nerves enter 

 the optic thalami, they cross one another in a chiasma, and 

 their fibres usually interlace as they cross. 



Alimentary system. The alimentary tract' exhibits 

 much division of labour, for not only are there parts suited 

 for the passage, digestion, and absorption of the food, but 

 there are numerous outgrowths, e.g. lungs and allantois, 

 which have nothing to do with the main function of the 

 food canal. 



By far the greater part of the food canal is lined by 

 endoderm or hypoblast, and is derived from the original 

 cavity of the gastrula the primitive gut or archenteron. 

 This is the mid-gut or mesenteron. But the mouth cavity 

 is lined by ectoderm, invaginated from in front to meet the 

 mid-gut. This region is the fore-gut or stomodaeum. 

 Finally, there is usually a slight posterior invagination of 

 ectoderm, forming the anus. This is the hind-gut or 

 proctodaeum, but it is practically absent in Vertebrates. 



Associated with the mouth cavity or stomodoeum are (a) teeth 

 (ectodermic rudiments of enamel combined with a mesodermic papilla 

 which forms dentine or ivory) ; (b) from Amphibians onwards special 

 salivary glands ; (<:) a tongue, a glandular and sensitive outgrowth 

 from the floor. The tongue develops as a fold of mucous membrane 

 in front of the hyoid, and afterwards becomes increased by growth of 

 connective tissue, etc. In larval Amphibians muscle strands find their 

 way into it, and Gegenbaur suggested that their original function was 

 to compress the glands. As they gained strength they became able 

 for a new function, that of moving the tongue. In all higher animals 

 (above Fishes), the nasal sac opens posteriorly into the mouth ; in 

 some Reptiles and Birds, and in all Mammals, the cavity of the 

 mouth is divided by a palate into an upper nasal and lower buccal 

 portion. 



The origin of the oral aperture is uncertain. In Tunicates it is 

 formed by an ectodermic insinking which meets the archenteron ; in 



