EMBRYOLOGY. 29 EMBRYOLOGY. Amnion and Allantoic. In the chick, as in reptiles and mammals, two special embryonic or- gans, in addition to the yolk-sue, make their ap- pearance early in development. These are the amnion and the allantois (q.V.). The amnion is an envelope formed about the embryo, which begins as a fold of ectoderm and somatic layer that completely surrounds the embryo. This fold becomes more and more elevated and finally, as a wall encircling the embryo, folds over the embryo, making a dome whose zenith aperture constantly diminishes until it dis- appears. The space between the embryo and the amnion is rilled with the "amniotic fluid.' The allantois is an out-pocketing from the hinder part of the food-canal which, growing through the still wide open body-cavity, spreads out against the egg membrane. It serves as a respira- tory organ, since blood-vessels ramify throughout it in great profusion. These get the oxygen which is admitted through the pores of the egg- shell, and carry it back to the embryo. As mammals, birds, and reptiles possess an amnion in their embryonic condition, these three classes have been united into a group called Amniota; while the term Anamnia is applied to the remaining vertebrates (amphibians and fishes) not so provided— the Sauropsida of Hux- ley. The Causes of Development. These are diffi- cult to discover. In the eighteenth century the idea prevailed that development consisted merely in the unfolding or growing larger of a minute individual, preformed in the egg or sperm-cell, likewise it was assumed that a new generation lay in the ovary of that little being, with still smaller inviduals inclosed within their bodies, ad infinitum. This is called the box-within- box theory. But exact study of the egg and the processes of embryological development show that such a miniature does not exist in the egg, and that development is not merely becoming larger. What is it that directs the course of de- velopment and determines at what point new organs shall arise? We do not at present know. It is generally believed that the course of de- velopment depends somehow upon the chemical constitution of nucleus and cytoplasm. As a piece of a hydra warps itself into an embryonic form and thereafter develops as the egg embryo develops, we must conclude that it is not so much the constitution of the egg or the sequence of cell-divisions that determines the form as some internal 'force' which acts throughout the entire organism. See Epigenesis ; Evolution (espe- cially remarks under Cooperative Evidences of Evolution) ; Fcetus; Gemmule; Germ Plasm; Ontogeny ; Preformation ; Recapitulation Theory; Vestigial Structures, etc. Bibliography. Korsehelt and Heider, Text- Boole of the Embryology of Invertebrates, vol. i., translated by Mark and Woodworth ; vols, ii.- iv., translated by Woodward (London and New York, 1895-97) ; Balfour. A Treatise on Compara- tive Embryology (London, 1881) ; Hertwig, Text- Book of the Embryology of Man and Mammals, translated by Mark (London, 1 S'J'it : Marshall, Vertebrate Embryology (London. 1893); Minot, Human Embryology (New York, 1892). EMBRYOLOGY, Human. Our knowledge of the development of the human embryo was in an exceedingly fragmentary condition until Wil- helm His, the distinguished German anatomist. published in 1885 his imiomi< Menschlicher Emhryoncn. This was the firsl and is yel the most important work on human embryology. Previously there existed, here and there, isolated descriptions of the first two months of pregnancy. The great dillicully in gelling embryos renders progress comparatively slow, since the principal sources of material for investigation are post- mortem examinations, operations, and abortions, and the embryologist is thus dependent on the courtesy of surgeons and physicians. Even now the number of good collections of human embryos is very small, the most important being those at Leipzig and Baltimore. In development, it is convenient to distinguish the three stages suggested by His: viz., the ovum, the embryo, and the foetus. The stage of the ovum embraces the first two weeks; the em- bryonal stage embraces the third, fourth, and fifth weeks, during which time the principal organs are developed; finally, in the foetal stage the em- bryonal features change to those of the foetus and full-term child. The average length of embryo and fcetus at various stages, is approximately shown by the following table: Length In inches In millimeters 0.1 0.2 0.3 0.4 to 0.5 0.6 0.8 1.0 2.0 3.9 7.9 11.8 14.6 16.7 19.7 3 " 4 " 8 11 to 12 16 5 " 6 " 7 " 25 4 " 5 " 6 " 7 " , 8 " 9 ■' Ovum Stage. There are no observations on normal ova of the first nine or ten days. It is evident from (he material of the later part of the ovum stage that there is an early and pre- cocious development of the chorion and villi. The voumjest mes known normal ovum Has de- scribed by Peters in 1899. It is ten to eleven days old and consists of a vesicle 3 X 1.5 X 1.5 mm. in size. (Fig. 1.) The vesicle is formed by the chorionic m e m - Inane, consisting of an outer layer of epithelial cells covered by nu- merous villi which are in con- tacl with the uterine wall of the mother, and an inner layer of mesenchyme. Attached to this inner layer at one side is the -mall embryo but .19 mm. in length. It is apparently simple in structure, consisting of an epithelial plate facing the small amniotic cavity. Fig. 1. section through ovum. Section through a portion of a human ovum, with an embryo 1.9 mm. long: am', amniotic cavity : *;, yolk-aae ; m. epithelial plate; e.cli, epithelium of chorion ; mes, niesen- chyma: en?, entoderm: sp, cleft in exoccelom. (After Peters.)
