vera,' or 'd. uteri.' Long and large villi extend from the chorion into the decidua, and at this period (latter half of the first month) there may be traced, upon its inner surface, orifices of canals that lead into the uterine sinuses. The maternal blood already flows freely into the maternal chamber, and, after passing everywhere among the villi, is returned into the uterine veins. Thus a temporary placenta is formed analogous to the diffused form described in Cetacea and certain Ungulata. But soon the villi increase in length and size on the side of the chorion next the uterine wall, and decrease on the opposite side, which becomes smooth or bald; this, pressing upon the hydroperionic space, finally obliterates it, and arrests the flow of blood to that part of the circumference of the chorion. On the other part, next the uterine wall, a circular space is left, like a meniscus, round the circumference of which decidual growths pass from the uterus to attach themselves to the chorion, and form the margin of the true placenta; then, as the uterus enlarges, concomitantly with the expansion of the ovum, a decidua, called 'serotina,' is reproduced to form the basis of the maternal placenta, from which septal processes extend grouping the developed villi of the chorion, or fœtal placenta, into lobes. With the further growth of the placenta these lobes become usually more and more confluent, the foetal also becomes more blended with the maternal part, until a structure results, as exposed in the section of the placenta and placental area of the uterus, fig. 582.

6

The line, u, u, indicates the extent of the uterine wall; ud is 'decidua serotina;' dp deciduous septa, p placenta, ch chorion, am amnion, of foetal blood-vessels, v, v villi, us uterine venous sinuses, a, a uterine curling arteries.' The two foetal arteries (allantoic or umbilical) communicate by a cross branch near the placental end of the funis, beyond which they spread in large branches over a considerable part of the free surface of the placenta, and subdivide dichotomously in the chorion, two or three times, before they penetrate the placental substance to ramify in the villous processes called 'placental tufts.' The stems of these are rooted in the chorion, and are tough and fibrous. Each tuft consists of an outer coriaceous and an inner soft tissue: a distinction which is continued to the terminal villi, fig. 583, as shown in the end of one from a stale placenta in which the inner vascular substance had shrunk away from the outer epithelial sheath, ib. b. From the third to the sixth month the arteries of the villi terminate in a rich capillary plexus at their periphery, ib. a. The veins from the capillaries unite

to accompany the arteries along the centre of the villi, emerge from the substance of the placenta, about sixteen in number,

Section of human uterus and attached placenta at 30th week of gestation. CCXLVI".

with a less tortuous course than the arteries, converging to the root of the funis and ultimately uniting to form a single umbilical

583

vein. After the sixth month the capillaries of the villi begin to disappear. The uterine arteries, fig. 582, a, about the size of a crow-quill in the later months, have a tortuous or curly course, and they ultimately pour their blood into the large venous sinuses, ib. us. These are most numerous upon the inner side of the decidua constituting the uterine surface of the placenta, passing obliquely through that layer into the uterine wall; some extend into the decid

ual septa, and some lead to the marginal channel termed the 'circular sinus'.

In Bimana the placenta is relatively thicker and smaller than in Quadrumana, and is attached to a relatively more contracted area of the womb than in the tailed kinds.

Fatus in utero, at the end of gestation; Human. CCXLVI".

At the end of pregnancy the fore part of the abdomen is occupied by the uterus, fig. 584, the foetus being commonly carried in the position there represented.

Nine months is the usual period of gestation in Bimana; but occasionally birth occurs at the eighth or even the seventh month, and the infant has been reared.

§ 407. Development of Mammalian Brain.-Limitation of space compels me to conclude this chapter with a brief notice of some of the more specially mammalian modifications of foetal formation.

The initial steps in the development of the nervous system of the Mammal closely correspond with those of the Reptile and Bird (vol. II. figs. 39, 135). The brain of the Kangaroo, a fortnight after birth, fig. 585, A, B, has not advanced beyond the condition of that of the embryo chick at the fourth day of in

585

g

B

Brain of new-born Kangaroo; magn.

5 times. LXXV.

cubation. Hanging motionless from the teat, like a foetus from the navelstring, its cerebellum, ib. A, c, has not transcended the filmy fold of the cold-blooded saurian type; but expansion has begun at the base, B, c, of what are destined to become the mammalian lateral lobes.' The mesencephalon constitutes the main part of the brain it is a large oblong vesicle,

in which the optic lobes, ib. d, begin to be faintly marked off from thethalamal' part, e, overlying the crura cerebri. No organ of the young air-breathing Marsupial offers a greater contrast to that in the new-born placental Mammal than the retarded brain. In form it has got no further than that in the six weeks embryo sheep, but it is firmer in texture: gradually advancing along the Mammalian route, its development stops at a certain point. The superincumbent mass of cerebellum expands, accommodating its ultimate sheet of grey matter to the cranial chamber by transverse folds; and the lateral lobes stretch out into appendicular lobes, fig. 74, e. The optic lobes, in their growth, show no disposition to special lateral expansion and divergence (as in the bird, vol. II. figs. 42, 44), but swell into a pair of closely united hemispheres: the special mammalian addition is due to growth of neurine in the fore part of the valvula vieussenii' between the processus a cerebello ad testes,' which proceeds in Marsupials and all higher Mammals to add a second pair of tubercles ('testes' of anthropotomy) to the optic lobes (nates' ib.). Into the cavity of the small hemispheric vesicles, fig. 585, g, i, the corpora striata' first bulge, and are soon followed by the hippocampal protuberances: with the former appear the transverse fibres of the anterior commissure, with the latter those of the hippocampal commissure. In Marsupials this is the sole addition to the transverse conneetions of the hemispheres common to lower Vertebrates: in Placentals, development of the commissural system proceeds to establish the supraventricular mass called 'corpus callosum.' But this is not necessarily accompanied by increased development of the cerebral lobes: the Lissencephala retain the lyencephalous LXXV. pl. vii. figs. 11, 12.

superficies and proportions of the superincumbent masses of the prosencephalon. In the Gyrencephala these extend backward over the mesencephalon, and more or less of the cerebellum: from the lissencephalous condition transitorily shown by the human fœtus, fig. 125, the middle lobes, d, progressively grow into posterior ones, finally extending in Archencephala above and beyond the cerebellum, and acquiring the proportions and conditions of the posterior horns of the lateral ventricles and hippocampi minores' peculiar to and characteristic of the human brain.

§ 408. Development of Mammalian Skeleton.-The notochord early begins to show a series of dilatations answering to the later intervertebral spaces. In the embryo head the blastemal coverings of the piers of the anterior cephalic hæmal arch (maxillaries) project freely, and appear as processes of the second (mandibular) arch: only the proximal parts of the third (hyoidean) arch are indicated by indentations, and the piers do not project freely. The chief developmental mammalian modification arises from the proximity of the precociously and rapidly growing appendages of the acoustic sense-organ ('ossicula auditûs') peculiar to the class accompanied with a reduction of the proximal part of the mandibular arch to the support of the tympanum, and with a slight forward dislocation of the distal part of the arch. In Monotremes the tympanic (vol. ii. fig. 197, 28), large and well-ossified in the blind and naked young, has its growth arrested and diverted by the rapid and excessive growth of the malleus, which becomes anchylosed to the tympanic by its long process, o, whilst its manubrium,' c, gives attachment to the radiating fibres of the muscle of the ear-drum. The incus, b, is represented by a small and early confluent epiphysis. The columelliform stapes d is relatively small as in other Mammals. The base of the mandible extends inwardly to join the tympanic, and its articular surface is also extended outward, as in the Bird: the conformity with the Chick in the relations of both tympanic and mandible to the primary and transitory cartilaginous hæmal arch, and the plain homology of the ossicle, b, with the better developed incus of higher Mammals, are decisive against the revival of Reichert's ill-founded conclusion as to the homology of the Mammalian incus with the os quadratum (tympanic) of Birds and Reptiles. In the mammary Kangaroo the tympanic, embracing by an upper bifurcation the hind part of Meckel's cartilage,' develops a convexity below adapted to the inner side of the