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

583

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

Foetus 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

B

Brain of new-born Kangaroo; magn.

5 times. LXXV'.

The

cubation. Hanging motionless from the teat, like a fœtus 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. 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 connections 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

1 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

6

.

ascending ramus of the mandible, and a smooth joint-like surface, fitting into the upper concavity of the inverted angle, answering to the persistent inner articular part of the condyle in birds. The fourth hæmal arch is close to the occiput in the Ruminant, and retrogrades as the neck is lengthened out by vertebræ interposed between head and chest. It retains, in Cetaceans,

almost the typical position exemplified in Fishes.

6

The common ossification of articular ends of bones from centres distinct from that of the shaft is a mammalian developmental characteristic. The ultimate confluence of the epiphyses' (vol. ii. p. 297) with the diaphysis' indicates maturity of growth: but in this relation there are differences in the same skeleton and in different species. In Man the epiphyses of the limb-bones toward which the arteriæ nutritiæ' run (p. 619) first coalesce with the shaft; those at the distal end of the humerus and proximal ends of the two antibrachials, e. g., at puberty, those at the opposite ends of the same bones at the twentieth year. The proximal epiphysis of the femur coalesces about the eighteenth year, the distal one at the twentieth; the proximal epiphysis of the tibia joins the shaft about the twenty-fifth year, the distal epiphysis five years earlier. The epiphyses of the vertebral bodies coalesce about the twenty-first year in Bimana, but they continue distinct for a much longer proportional period of life in Cetacea. Epiphyses and short bones of limbs, those of the carpus and tarsus, e. g., continue cartilaginous some time after the shafts of the long bones are ossified, as shown in fig. 586. This figure also exemplifies the early manifestation of ordinal characters; the inner digit of the pelvic limb, in the foetal Monkey (Cercopithecus

586

b

sabaus) already shows by its relative shortness and divergence from the others that it is destined to oppose them, and to terminate the member by a prehensile hand: while, from the earliest manifestation of the digits of the same limb in the human embryo, the hallux' by its proportions and parallelism with the other toes indicates the destination of the answerable part to become a plantigrade foot, perfected to sustain and move the body of an erect Biped.

From foetal lower limb; nat. size. a, Monkey. b, Man. CCLXXVII".

§ 409. Membrana pupillaris.—The differences in degree of indi

1 ccxcv". p. 727.