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the length of the embryo is 1.9 mm., the neural folds are well developed, the neural groove is still completely open, and six pairs of mesodermal somites are visible (Fig. 97).
In the next few days the length increases to 2.5 mm., the neural groove closes except in the cranial and caudal regions, the number of mesodermal somites is increased to fourteen pairs, and the cranial region begins to bend ventrally as the cervical flexure forms (Fig. 98).
By the end of the first month the greatest length of the embryo is about 2.6 mm., the head is bent at right angles to the body, the Wolffian ridges have appeared along the ventral margins of the mesodermal somites and indications of the limb rudiments
FIG. 95. FRASSI'S ZYGOTE. Estimated to be 18-19 days old (Bryce). The embryonic area is 1.17 mm. long and 6 mm. broad. Copied from Normaltafeln, Keibel and Elze, representing a reconstruction. The chorion is not shown. The upper part of the amnion is cut away, and the dorsal aspect of the embryonic area is seen from above. In the centre of the area is the neurenteric canal and caudal (inferior in the Fig.) to it is the primitive groove. Cephalwards of the neurenteric canal is the neural groove, in the middle of the neural plate. At the lower (caudal) end of the Fig. is seen a section of the body stalk containing the allantoic diverticulum, and the nodulated area seen at the upper and right lateral part of the Fig. is a portion of the yolk-sac.
FIG. 96.-SPEE'S ZYGOTE. (From Keibel and Elze's Normaltafeln.) Length of embryonic area 1.54 mm. Estimated age 19-20 days (Bryce). At the lower end of the Fig. (caudal end of the embryo) is seen a portion of the chorion attached to the embryo by the body stalk. A portion of the amnion is still attached to the margin of the embryonic area, and the dorsal surface of the embryonic area is exposed. In the median plane of the area is the neural groove, and at the caudal end of the groove is the neurenteric canal. The caudal part of the area is bent ventrally, and upon it is the remains of the primitive groove. The yolk-sac is seen at the upper and right part of the Fig.
are present. The rudiments of the otic vesicles have appeared as slight depressions in the region of the hind-brain. The anterior and posterior neuropores are still open (Fig. 99).
In the latter part of the fourth or the beginning of the fifth week the embryo attains a length of about 5 mm., when measured from the vertex of the head to the base of the tail, the mesodermal somites increase to thirty-five; the rudiments of the fore- and hindlimbs become quite distinct; the otic vesicles sink into the interior of the head but remain connected with the surface by the recessus labyrinthi, the tail becomes a very definite appendage, and the bulgings caused by the otic vesicles are quite obvious on the surface of the head. The cervical flexure remains acute, and the head bends at right angles upon itself in the region of the mid-brain, forming the cephalic flexure, with the result that the frontal extremity of the head is turned caudally (Fig. 100).
By the end of the fifth week the length of the embryo has increased to 11 mm. (CR) 1 1 CR indicates the crown-rump or crown-breech measurement which corresponds with the sitting height (Mall).
(Mall). Forty-three mesodermal somites are present, but only about twenty-one are visible on the surface. During the fifth week the lens of the eye appears as a thickening of the surface ectoderm; sinks into the interior of the eyeball; becomes a vesicle and separates from the surface. The three segments of the fore-limb become visible, and the rudiments of the fingers appear. The hind-limb is less advanced; the thigh segment is not distinct, and the rudiments of the toes are not yet visible. The third and fourth visceral arches disappear from the surface and lie in the depths of the precervical sinus, a depression between the neck and the anterior part of the body; this is overlapped, superficially, by the caudal margin of the second arch, which grows tailwards and forms the operculum of
the sinus (Figs. 101, 102). During the fifth week the head grows rapidly, and becomes relatively very large as contrasted with the body.
During this week also the olfactory pits appear, and grow dorsally in the roof of the stomatodæum, separating the median from the lateral nasal processes; the median process is divided into the two globular processes; and the maxillary processes of the mandibular arches, growing towards the median plane, fuse with the lateral nasal and the globular processes, so completing the lateral parts of the primitive cranial lip (Figs. 64, 65, 66).
The nodular outgrowths which form the rudiments of the auricles appear on the margins of the hyo-mandibular cleft and fuse together, and by the end of the week traces of the tragus, the helix, and the antitragus are visible (Fig. 103).
By the seventh week the embryo has attained a length of 17 mm. (CR). flexure has begun to unfold. The rudiments of the eyelids have appeared. processes have fused together, but there is still a distinct notch in the cephalic or upper lip. The margins of the auricles are now well defined;
The cervical The globular middle of the the hands are
102.-EMBRYO, 7-2 mm. (CR), and 8 mm. greatest length. From Keibel and Elze's Normaltafeln.) The limbs have begun to fold ventrally. The second arch has completely overlapped the third and fourth which now lie in the precervical sinus, and the sinus still opens on the surface at the posterior border of the second arch. The lens of the eyeball is very evident, and rudiments of the auricle of the external ear have appeared on the mandibular or first, and the hyoid or second arch.
FIG. 101. EMBRYO OF 72 MM., CR MEASUREMENT. 8.5 mm. greatest length. (From Keibel and Elze's Normaltafeln.) The fore-limb is distinctly in advance of the hind-limb. The second branchial arch has begun to overlap the third and fourth and to enclose the precervical sinus. The tip of the maxillary process is in contact with the lateral and medial nasal processes at the margins of the olfactory pit.
By ARTHUR THOMSON, F.R.C.S.
THE term skeleton (from the Greek, σkeλerós, dried) is applied to the parts which remain after the softer tissues of the body have been disintegrated or removed, and includes not only the bones, but also the cartilages and ligaments which bind them together. In the restricted sense of the word the skeleton denotes the osseous framework of the body. It is in this sense that it is generally employed in human anatomy.
The skeleton serves to support the softer structures which are grouped around it, and also affords protection to many of the delicate organs which are lodged within its cavities. By the articulation of its several parts, its segments are converted into levers which constitute the passive portion of the locomotory system. Recent research has also proved that certain cells found in bone-marrow are intimately associated with the development and production of some of the corpuscles of the blood.
Bone may be regarded as white fibrous tissue which, having become calcified has undergone subsequent changes, so as to be converted into true osseous tissue. Most probably all bone is of membranous origin, but it may pass through a stage in which cartilage plays an important part in its development. In many instances the cartilage persists, and is not converted into bone, as in the case of the articular cartilage which clothes the joint surfaces, the nasal septum, the cartilages of the nose, and the cartilages of the ribs. A persistence of the membranous condition is met with in man in the case of the tentorium cerebelli, which in some groups of animals (Carnivora) is converted into a bony partition.
Skeletal structures may be derived from each of the three layers of the trilaminar blastoderm. The exo-skeleton includes structures of ectodermal, and some of mesodermal origin, in the shape of hair, nails, feathers, teeth, scales, armourplates, etc., whilst the endo-skeleton, with which we are more particularly concerned, is largely derived from the mesodermal tissue, but also includes the notochord, an entodermal structure which forms the primitive endo-skeleton, around which the axial skeleton is subsequently developed in the Vertebrata. The endo-skeleton is divisible into an axial portion, appertaining to the trunk and head, and an appendicular part, associated with the limbs. It also includes the splanchnic skeleton, which comprises certain bones developed in the substance of some of the viscera, such as the os cordis and os penis of certain mammals. In man, perhaps, the cartilaginous framework of the trachea and bronchi may be referred to this system.
The number of the bones of the skeleton of man varies according to age. Owing to a process of fusion taking place during growth, the number in the adult is less than the number in the child. The following table does not include the sesamoid bones, which are frequently developed in tendons, the most constant ossicles of this description being those in relation to the metacarpo-phalangeal joint of the thumb, and the metatarso-phalangeal joint of the great toe.