VERTEBRATA. 427 
forms of nutrition in regular sequence. They are as 
follows :— 
1. FREE OR LarvaL Nutrition.—This is found at a 
very early stage in Amphioxus and later in fishes and 
Amphibia. In it the larva or young form catches its 
own food with mouth and ingestive organs. It is practically 
the only mode of nutrition adopted by Amphioxus. 
2. YOLK or LeciTHAL Nutrition.—The young form 
is supplied by the parent with an inert mass of yolk or 
fatty material, and whilst the yolk lasts it is mainly enclosed 
in the egg-membrane and is known as an embryo instead 
of a larva. The yolk is stored primarily in the alimentary 
canal which causes the latter to protrude as a large bag 
or sac called the yo/k-sac. In certain fishes and Amphibia 
the lecithal form of nutrition is succeeded directly by the 
larval nutrition, the mouth and other ingestive organs be- 
coming functional at the completion of yolk-absorption. 
In other words, the young frog, for example, is supplied 
with yolk till shortly after hatching, when the mouth opens 
and a vegetable diet is then resorted to. 
The lecithal form of nutrition culminates in elasmobranch 
fishes, in Sauropsida and in Monotremata amongst mammals. 
Like the larval nutrition, it is entirely given up in the rest 
of the Mammalia. 
3. ALBUMINAL NutTRITION.—In Amp/ibia, such asthe frog, 
the egg itself is surrounded by a clear hyaline mass of an 
albuminous substance which swells up after oviposition and 
serves as a protection to the embryo. It does not appear 
in the frog to be used as nutriment. but in the Sauropsida, 
e.g. Chick, the same material surrounds the true egg as a 
mass of albumen between it and the shell. As in the frog, 
this material is produced by a series of glands in the lower 
part of the oviduct. Here, however, the albumen is not 
“required for protection as this function is performed by the 
shell, but it is absorbed by the embryo towards the later 
days of incubation when the lecithal nutrition is terminating. 
Little is known about the absorption of this albumen. The. 
serosa may play some part, but the basal part of the yolk-sac, 
in contact with it, is said to become the absorbing area, and 
the nutriment would thus find its way to the embryo through 
the medium of the yolk-sac. Little is known concerning 
