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, eg. 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 stomodzum. 
Finally, there is usually a slight posterior invagination of 
ectoderm, forming the anus. ‘This is the hind-gut or 
proctodzum, but it is practically absent in Vertebrates. 
Associated with the mouth cavity or stomodzum are—(a) teeth 
(ectodermic rudiments of enamel combined with a mesodermic papilla 
which forms dentine or ivory); (4) from Amphibians onwards special 
salivary glands; (c) 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 
