VERTEBRATA. 429 
the albuminal nutrition of the mammals, though in the 
Metatheria, at least, it appears to be an important factor in 
the nourishment of the young, and in many Luwtheria, in 
which the lecithal nutrition is entirely replaced, it probably 
plays an important réle. There are numerous glands of 
the oviduct, uterine glands, which probably secrete the 
albumen. The albuminal nutrition is therefore the second 
form of nutriment supplied by the parent to the embryo. 
4. LacrgeaL Nutrition. — This is the production of 
“milk” by mammary glands. The “milk” is elaborated -by 
skin-glands and is supplied, not to the embryo, but to the 
young animal after birth; hence no special organ beyond the 
mouth and alimentary canal is needed. Traces of this form 
of nutrition are found in the Sauropsida (pigeon’s “ milk”) 
and in the Prototheria, but it attains its greatest develop- 
ment in the AMezatheria, in which it follows very closely 
upon the albuminal nutrition. It is found usually in the 
Eutheria, but is in them being replaced by the last method 
of nutrition. 
5. H@mat Nutrition.—In this form the young animal 
feeds directly upon the blood of the mother by absorption 
through the blood-vessels of the yolk-sac and of the allantois. 
The maternal vessels form with those of these two organs a 
complex vascular organ called a placenta. The yolk-sac 
placenta is found in MMe¢atheria and, exceptionally, the 
allantoic, but neither is sufficiently elaborated to replace to 
any extent the lacteal; whereas in the Hwtherta this heemal 
nutrition is much the most important, though preceded by 
an albuminal and succeeded by a lacteal. 
Foetal Membranes.—The distension of the ventral 
wall of the body by an accumulation of yolk produces a 
large sac-like protuberance of the intestine, called the yo/k- 
sac, covered by the distended body-wall forming the serous 
membrane or serosa. In the Ammnio¢a other two foetal mem- 
branes are found. The amnion is a protective membrane 
produced from the serosa and similarly formed of epiblast 
.and somatic mesoblast, whilst the a//anfois is a median 
ventral process of the intestine and is similarly formed of 
hypoblast and splanchnic mesoblast. In Sauropsida the 
allantois acts as a urinary bladder and a respiratory organ, 
