Nutrition of Embryo.— Throughout the entire course of development the mechanical effect of food-yolk is very marked, not only in its retardation or prevention of cleavage in cer tain parts of the egg but in its mechanical ef fect on the formation of the germ layers and its physiological relation to development of the nutritive system. In general, though there are many exceptions, large eggs rich in yolk de velop slowly and the resulting embryos hatch in an advanced state, often with essentially the adult form, while small eggs poor in yolk must early develop some means of securing food and usually in such cases the embryonic period is very brief, the embryo hatching in the form of a larva, often totally different from the adult. Such larva are especially common among ma rine invertebrates, in which usually they have the form of minute free-swimming organisms, often with no resemblance to the adult either in form or habit. Examples are the trocho phores of annelids and molluscs, the nauplius of the crustacean, bipinnaria of the starfish, etc. In some cases the larva represents only a small portion of the future adult animal, occa sionally only a portion of the head precociously equipped with an alimentary system and means of locomotion. These larva feed on various micro-organisms and eventually •become made over into the adult form by a more or less com plete metamorphosis.
Extra-embryonic Membranes.— In some animals extra-embryonic membranes are pro duced which subserve a temporary function in the protection or nutrition of the embryo and which are lost at hatching or at birth. In the higher vertebrates such structures include the chorion, a membrane forming the outer wall of the entire embryonic vesicle; the amnion, a closed water-sac lined with ectoderm and com pletely enclosing the embryo, and the allantois, an extension outside the body of the urinary bladder which in reptiles and birds and also in the primitive egg-laying mammals known as monotremes spreads its vascular wall inside the chorion close to the porous egg shell and serves physiologically as an embryonic respiratory organ. In the marsupial mammals, such as the kangaroo and opossum, the young are nourished during the very brief period of gestation by ((uterine milk," a secretion of uterine glands which the embryo absorbs by means of its vas cular membranes, chiefly the yolk sac. Uterine milk is also an important source of nutriment to the embryo even in some placental mammals, where it contains leucocytes and the detritus of disintegrated cells of various sorts in addition to glandular secretion. The placental mammals are so called because an organ named the placenta is developed, which is essentially com posed of villi or vascular tufts developed on the surface of the chorion and supplied with blood vessels by the allantois. These chorionic villi come into intimate contact with the mucous membrane lining the uterus, which becomes profoundly modified during pregnancy, and through their rich vascular supply the blood of the embryo is brought into close osmotic rela tion with that. of the mother, thus permitting
the diffusion into the embryonic circulation of soluble foods and oxygen from the maternal blood and at the same time removing carbon dioxide, so that the placenta serves the func tions of embryonic nutrition and respiration. It is important to note that there is no admix ture of maternal and embryonic blood, the two in all cases being separated by an osmotic mem brane. In some mammals the placental villi are minute and scattered over almost the entire chorion, forming what is known as a diffuse placenta, as in the horse and pig. In others they are aggregated' into a number of brush like tufts, the cotyledonary type, found in most of the ruminants, as the deer, ox, etc. In other cases the milli are limited to a broad girdle forming a zonary placenta, as in most carniv ora and in the elephant. In insectivores, rodents and most of the primates, including man, a discoidal type of placenta is found in which the villi are limited to a single cake like mass. In many cases, especially the diffuse and cotyledonary types, the villi fit into cor responding crypts of the uterine mucous mem brane from which they are drawn at birth without injury. In other cases, the so-called deciduate placentae, the uterine lining becomes greatly altered and its union with the villi be comes so firm that at birth portions of the maternal tissue are torn away with the em bryonic placenta. Recent investigation, how ever, favors the view that even in deciduate types the maternal portion is largely absorbed before full term, so that the placenta at birth is almost wholly of foetal origin. Though the eggs of mammals are very minute and undergo complete cleavage, their development is re markably like that of the large eggs of reptiles and birds in the mode of germ-layer formation, organogeny and relations of extra-embryonic i membranes, and strongly indicates the descent of mammalia from ancestors whose eggs had abundant yolk and underwent partial cleavage. This, indeed, is often cited as one of the classsic examples of the persistence of ancestral devel opmental habit. In this particular case the evi dence is confirmed by the occurrence of ovip arous habit, large eggs and reptile-like mode of development in the primitive monotreme mammals.
Bibliography.—Among numerous works on embryology the following may be mentioned as of especial value : Balfour, F. M., (A Treatise on Comparative Embryology) (Vols. I and II, London 1885) ; Ziegler, H. E., 'Lehbruch der vergleichenden Entwickelungsgeschichte der niederen Wirbelthiere> (Jena 1902) ; Lillie. F. R., 'Development of the Chick> (New York 1908) ; Kellicott, W. E., 'General Embryology) (New York 1913) ; Jenlcinson, J. W., !Verte brate Embryology) (Oxford 1913) ; MacBride, E. W., 'Textbook of Embryology) (Vol. I, Invertebrates, London 1914) ; Hertwig, 0., (Elemente der Entwickelungsgeschichte: Ver tebrates) (5th ed., Jena 1915) ; Marshall, F. H. A., 'The Physiology of Reproduction) (London 1910).