DEVELOPMENT OF THE BRAIN AND VESSELS OF THE BODY 395 



nourishment by a vital osmosis ; the pabulum being, as indicated, mainly supphed by the Hquor sanguinis, by the 

 uterine glands and secretions of the mother. As yet the ovum reveals no internal structure. Very soon the process 

 of segmentation of the vitellus and the formation of the blastodermic and other membranes begin. This process has 

 been already described, and need not be repeated. In the human embryo and foetus various and important accessory 

 structures and fluids are developed and produced. The chief of them are the chorion, amnion, and their special fluids ; 

 the umbihcal vesicle, the allantois, the umbihcal cord, and the foetal portion of the placenta. They are seen at 

 Plate xcii., Fig. 1, page 396. 



PLATE XCII 



Plate xcii. illustrates the development of the impregnated human ovum in its several stages as embryo and 

 foetus ; also the development of the brain, blood-vessels, &c. 



Fig. 1.— a to L inclusive. Represent changes occurring in the impregnated human ovum during gestation (after Crusius). 



A. Human ovum two weeks old, natural size. 



B. Human ovum two weeks old, twice natural size, a. Ovum embedded in mucous lining of uterus, namely, the decidua 

 vera (b, b), decidua serotina (c), and decidua reflexa (rf) ; e, villi of chorin. 



0. Human embryo three weeks old, half natural size, a, Body of embryo ; b, decidua ; c, amnion ; d, chorin ; e, villi of chorin ; 

 /, umbilical vesicle. 



D. The same as 0, three times natural size (lateral view), a, b, c. Anterior, middle, and posterior brain ; d, eye vesicle ; 

 e, ear vesicle ; /, visceral arches ; g, navel; h, primitive segments (vertebral column) ; i, amnion ; ;, allantois ; k, umbilical vesicle. 



E. The same as D, still more magnified (frontal view), a, b, Frontal and mid-brain ; d, eye vesicle placed laterally ; i; aperture 

 of mouth ; s, maxillary process ; /, the four visceral arches. By the third week the embryo measures one-sixth of an inch in 

 length and is covered with villi ; the allantois reaches the chorion ; the primitive circulation is established, the back is curved, and 

 visceral arches appear. Traces of primary brain, eye and ear vesicles are distinguishable. The alimentary canal occurs as a 

 straight tube and the mouth as a five-sided orifice. 



F. Human embryo tour weeks old, natural size, a, Body of embryo ; r, amnion ; d, chorion with villi (e) ; /, cavity of chorion ; 

 (J, navel ; h, umbilical vesicle. 



By the fourth week the embryo measures one-third of an inch in length. The chorion is wholly covered with villi and the 

 umbilical vesicle fully developed. The eyes are seen as two black dots, the superior and lower extremities appearing as buds. 

 The proto-vertebrse (future vertebral column) are also well marked, and terminate in a caudal portion or tail. 



G. Human embryo four weeks old, three times natural size, a, Head with eye, ear, and anterior, middle, and posterior brain ; 

 /, visceral arches ; i, amnion ; m, superior and inferior (n) limbs appearing as buds ; h, h', rudimentary column ending in a caudal 

 portion ; o, foetal portion of placenta ; k, umbilical vesicle. 



H. Human uterus with embryo five weeks old (semi-diagrammatic), u, a, a, Walls of uterus ; b, cavity of uterus ; c, os uteri ; 

 d, cervix uteri with plug of mucus to seal the entrance ; e, decidua vera ; /, decidua reflexa ; g, decidua serotina ; h, villi of chorion. 

 The embryo is seen enlarged at I. 



1. Human embryo five weeks old, twice natural size, a, Head with face, eyes, ears assuming shape ; c, amnion invests embryo ; 

 m, n, superior and inferior extremities growing, fingers and toes forming ; o, foetal portion of placenta with villi composed of fine 

 loops of blood-vessels ; p, allantoic vein ; r, chorion ; h, yolk sac and vitellus ; I, vitelline stalk. 



J. Human foetus seven weeks old, half natural size. (The term fcetus takes the place of embryo when the parts assume shape.) 

 I', Chorion ; s, villi of chorion ; t, blood-vessels of umbilical cord spreading on the inside of the chorion ; u, amnion enveloping 

 fcetus : V, body of foetus ; ir, umbilical vesicle. 



K. Human foetus eight weeks old. The shape of the body is now clearly indicated, a, b, c, anterior, middle, and posterior 

 portions of head and brain ; d, eyelids forming ; e, ear forming ; m, superior and inferior (») extremities show main divisions of 

 limbs, and the hands and feet fairly formed ; .'', body of uterus assuming shape, the caudal portion diminishing. 



L. Human fcetus twelve weeks old within the amnion, half natural size, u, Amnion with fluid investing the foetus and guard- 

 ing it against accident ; /■, chorion ; o, o, villi of chorion with fine loops of blood-vessels forming fatal portion of placenta ; x, body 

 of "foetus well formed ; head, face, eyes, ears, and superior and inferior extremities well defined. 



Note. — For placental and foetal circulation, see Plate xcv., Figs. 1 and 3, p. 407. 



Fig. 2. — Plan of section of uterus with fully formed ovum (human), g, Mucous lining or membrane of uterus, opposite placenta 

 (decidua serotina of authors) ; '), c, lining membrane on body of uterus (decidua vera of authors) ; d, lining membrane of fcetus 

 (decidua reflexa of authors). This membrane (e) is found on the free surfaces of the chorionic villi (a), being in fact the mucous 

 lining of the chorion. Such parts of it as are not engaged in covering the chorionic villi become thinned away and disappear 

 towards the full term. It may, however, always be found on the free or uterine surface of a normal placenta. .:(., Chorionic villi 

 constituting foetal portion of placenta (the maternal portion is formed by capillary vessels found in mucous lining, ;/) ; h, chorion, 

 from which the chorionic villi spring ; /, amnion ; i, umbilical vesicle ; ;, Fallopian tube. 



Note.— Aa pregnancy advances the parts marked c, ii, ;/ approach each other, and become accurately apposed. The same holds 

 true of the parts marked b and d. This apposition and blending of maternal and fcetal structures facilitates the exchange (chiefly 

 by osmosis) of nutritive and eflfete materials between parent and oft'spring (the Author). 



Fig. 3.— Face or front views of various embryos as seen in man (A), in the cat (B), in the sheep (C), and in the bat (D) (after 

 Haeckel). These resemble each other, and bespeak a general plan as represented by types and as apart from and independent of 

 evolution. 



Fig. 4. — Circulatory systems of the fcetus. , . , , ■ • c ii 



A. Truncus arteriosus, with one pair of aortic arches (a, a'), and dotted Imes indicatmg the future position of the second and 

 third pairs of aortic arches. These are numbered 1, 2, 3. b, Thoracic aorta. • i m • i 



B. Truncus arteriosus with four pairs of aortic arches (a, a'), and indications of the fifth aortic arch. Ihe aortic arches are 

 numbered 1, 2, 3, 4, 5. b, Thoracic aorta ; i; external carotid ; d, internal carotid. , c , , 



C. Truncus arteriosus with three posterior pairs of aortic arches («, o') from which the permanent vessels of the embryo are 

 developed, with dotted lines showing the position of the two (now) obliterated anterior arches (6, //). c, External carotid ; d, in- 

 ternal carotid ; e, thoracic aorta ; /,/, vertebral arteries ; g, g', axillary arteries ; h, subclavian arteries. 



