VER TEBRA TA. 429 



the albuminal nutrition of the manimals, though in the 

 Metatheria^ at least, it appears to be an important factor in 

 the nourishment of the young, and in many Eutheria^ in 

 which the lecithal nutrition is entirely replaced, it probably 

 plays an important role. 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, Lacteal 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 Metatheria, 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.EMAL 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 Metatheria and, exceptionally, the 

 allantoic, but neither is sufficiently elaborated to replace to 

 any extent the lacteal ; whereas in the Eutheria this haemal 

 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 yolk- 

 sac^ covered by the distended body-wall forming the serous 

 membrane or serosa. In the Amniota 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 allantois 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. 



