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lies dorsal to the pericardium, and between the fore-gut medially and the body laterally, is still a pleuro-pericardial canal. The Separation of the Pericardial, Pleural, and Peritoneal Parts of the
Colom.- In the lateral wall of each pleuro-pericardial canal, near its
cephalic end, lies the duct of Cuvier, Alimentary canal
passing towards the heart; and a
Commencing lateral part of diaphragm Septum transversum
Pleuro-peri. cardial canal
B Fig. 91.-SCHEMA OF LATER STAGE OF DIFFERENTIATION
OF CELOM. A, from above. B, transverse section cut FIG. 92.-SCHEMA OF A TRANSVERSE SECTION level of lung bud in A.
AT THE LEVEL OF THE LUNG BUD IN FIG. 91.
pericardial canal (Fig. 91). As the lung buds grow the cavities of the pleuropericardial canals increase in size, and each passes ventrally, round the side of
the pericardium towards the ventral wall of the body, until it is
separated from its fellow of the opAlimentary canal
posite side only by a median mesoderm-filled interval, which becomes the anterior mediastinum and the
anterior part of the superior mediaPleural cavity
stinum (Fig. 94). At the same time Closed aperture between the cavity of each pleuro-pericardial pleura and pericardium
canal, and the growing lung bud
in its interior, grow towards the
cephalic end of the embryo (Fig.
Duct of Cuvier
93). As it passes cephalwards the growing lung lies to the lateral side of the duct of Cuvier, which is thus forced against the cephalic end of the pleuropericardial canal, compressing it towards the median plane, against the sides
of the trachea and the @sophagus, until its cavity is obliterated. When this occurs the pericardial cavity is entirely shut off from the remainder of the celom, and it becomes a completely closed space (Fig. 93).
As the closure of the pericardial cavity is taking place two wing-like folds of mesoderm, connected ventrally with the septum transversum and laterally with the body walls, appear, caudal to the lungs (Figs. 91, 93). These folds are the rudiments of the lateral parts of the diaphragm, and each passes medially until it fuses with the mesoderm of the side wall of the fore-gut and with the dorsal mesentery. When this fusion is completed the cavity of the portion of the colom surrounding the lung, the original pleuro-pericardial canal, is separated from the more caudal part of the colom, which now becomes the peritoneal cavity.
Only the broad outlines of the processes by which the pleuro-peritoneal canals are separated from the pericardium and the peritoneum are mentioned in the preceding paragraphs. The details of the processes are too complicated for description in an ordinary text-book of anatomy.
The Formation of the Diaphragm.— There are four main parts of the diaphragm, a ventral, a dorsal, and a right and a left lateral.
The ventral part is formed from the septum transversum, which is gradually differentiated into a caudal, an intermediate, and a cephalic part. The caudal part is transformed into (1) the mesodermal tissue of the liver, which grows towards the abdomen, (2) the falciform and coronary ligaments, and (3) the small omentum. The cephalic part becomes the caudal or diaphragmatic wall of the pericardium. The intermediate part is transformed into the ventral portion of the diaphragm.
The dorsal part of the diaphragm is developed from the mesoderm of the dorsal mesentery of the fore-gut. Each lateral part is derived from a lateral ingrowth which springs ventrally from the septum transversum and laterally from the body wall. The two lateral portions grow towards the median plane till they fuse with the dorsal portion; but in some cases, especially on the left side, the fusion is not completed. In such cases an aperture of communication remains, between the pleural and the peritoneal cavities, through which a portion of the abdominal contents may pass into the pleural sac, constituting a diaphragmatic hernia.
SUMMARY OF THE EXTERNAL FEATURES OF THE HUMAN EMBRYO
AND FETUS AT DIFFERENT PERIODS OF DEVELOPMENT.
During the first fourteen days after the impregnation of the ovum the human zygote descends through the uterine tube, assumes the morula condition, enters the uterus, penetrates into the decidua compacta, and differentiates into three vesicles and a mass of primitive mesoderm ; but, probably, it is not until the beginning of the third week, if Bryce's calculations are correct, that a definite embryonic area is present. By that time the zygote is an ovoid vesicle measuring 2:4 by 1.8 mm. Its wall is formed by the trophoblast, and it contains two inner vesicles, the ecto-mesodermal and the entodermal vesicles. The inner vesicles are surrounded by a mass of primary mesoderm in which the extra-embryonic portion of the cælom is beginning to appear. At this period the embryonic area is the region where the walls of the two inner vesicles lie in relation with one another, and it is •19 mm. long (Fig. 30).
By the eighteenth or nineteenth day the area has attained a length of 1:17 mm. and it is ·6 mm. broad. It is pierced, about the centre of its length, by the neurenteric canal ; the primitive streak has appeared on the dorsal surface of the area ; the primitive groove is distinct, and the neural groove is indicated. The body-stalk is bent dorsally, at right angles with the area, and it contains the allantoic diverticulum, which has already been projected from the wall of the entodermal vesicle (Fig. 95).
During the next twenty-four hours the length of the embryonic area increases to 1.54 mm.; the neurenteric canal is moved caudally, to a point well behind the middle of the length of the area, and the posterior part of the area is bent ventrally, forming the posterior boundary of the hind-gut region and indicating the position of the future cloacal membrane. The head fold has begun to form, and the pericardial region lies in the ventral wall of the rudimentary fore-gut (Fig. 96).
By the middle of the third week the head and tail folds are distinctly formed and 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 NorFaltafeln, 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 bindlimbs 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)
CR indicates the crown-rump or crown-breech measurement which corresponds with the sitting beight (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). The cervical flexure has begun to unfold. The rudiments of the eyelids have appeared. The globular processes have fused together, but there is still a distinct notch in the middle of the cephalic or upper lip. The margins of the auricles are now well defined; the hands are