VERTEBRATA. 427 



forms of nutrition in regular sequence. Tliey 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 yolk-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 Nutrition. — Yxv Amphibia, such as the 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 



