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folded medially; the tips of the fingers are free, and the palms rest on the cranial part of the distended abdomen. The thighs and the toes have appeared, and the tail has begun to fuse with the caudal end of the body (Fig. 104).
At the end of the eighth week, when the embryo becomes a foetus, it has attained a length of about 25 mm. (CR). The auricles project from the sides of the head, the tail has almost disappeared from the surface, and the toes are free from one another. The cervical flexure is now very slight, and although the head is still relatively large, the disproportion between it and the body has begun to decrease (Fig. 105).
Third Month.-The head grows less rapidly, and, though it is still large, it is relatively smaller in proportion to the whole body. The eyelids close, and their margins fuse
together. The neck increases in length. The various parts of the limbs assume their definite proportions, and nails appear on the fingers and toes. The proctodæum is formed and the external generative organs are differentiated, so that the sex can be distinguished on external examination. The skin is a rosy colour, thin and delicate, but more consistent than in the preceding stages. By the end of the third month the total length of the foetus, excluding the legs, is 7 cm. (2 in.), including the legs, 9-10 cm. (33-4 in.), and it weighs from 100-125 grammes (3-4 oz.).
Fourth Month.-In the fourth month the skin becomes firmer, and fine hairs are developed. The disproportion between the fore- and hind-limbs disappears. If the fœtus is born at this period it may live for a few hours. Its total length from vertex to heels is 16-20 cm. (6-8 in.), from vertex to coccyx 12-13 cm. (44-5 in.), and it weighs from 230-260 grammes (81-91 oz.).
Fifth Month. The skin becomes firmer, the hairs are more developed, and sebaceous matter appears on the surface of the body. The legs are longer than the arms, and the umbilicus is farther from the pubis. At the end of the month the total length of the
fœtus, from vertex to heels, is 25-27 cm. (10-10 in.), from vertex to coccyx 20 cm. (8 in.), and its average weight is about half a kilogramme (1 lbs.).
Sixth Month. The skin is wrinkled and of a dirty reddish colour. The hairs are stronger and darker. The deposit of sebaceous matter is greater, especially in the axilla and groins. The eyelashes and eyebrows appear. At the end of the month the
total length of the foetus, from vertex to heels, is from 30-32 cm. (12-12 in.), and its average weight is about one kilogramme (23 lbs.).
Seventh Month. The skin is still a dirty red colour, but it is lighter than in the previous month. The body is more plump on account of a greater deposit of subcutaneous fat. The eyelids re-open, and the foetus is capable of living if born at this period. Its total length at the end of the month, measured from vertex to heels, is 35-36 cm. (14-14 in.), and its weight is about one and a half kilogrammes (33 lbs.).
Eighth Month. The skin is completely covered with sebaceous deposit, which is thickest on the head and in the axillæ and groins, and its colour changes to a bright flesh tint. The umbilicus is farther from the pubis, but it is not yet at the centre of the body. The total length of the fœtus, from vertex to heels, is 40 cm. (16 in.), and its weight varies from 2 to 2 kilogrammes (4-51⁄2 lbs.).
Ninth Month. The hair begins to disappear from the body, but it remains long and abundant on the head. The skin becomes paler, the plumpness increases, and the umbilicus reaches the centre of the body. At the end of the ninth month, when the fœtus is born, it measures about 50 cm. from vertex to heels (20 in.), and it weighs from 3-3 kilogrammes (6-7 lbs.).
The age of a fœtus may be estimated, approximately, by Hasse's rule, viz., Up to the fifth month the length in centimeters, the lower limbs being included, equals the square of the age in months, and after the fifth month the length in centimeters equals the age multiplied by five.
NOTE 1.-Evidence is gradually accumulating which tends to show that the reduction of the number of chromosomes may take place during the last divisions of the germ mother cell, that is before the growth of the oocyte or spermatocyte I commences, and therefore before maturation commences.
NOTE 2.-There is evidence which points to conclusions somewhat different from those stated on p. 14, regarding the dentoplasm in mammalian ova, but it is not yet sufficient or sufficiently conclusive to justify its incorporation in a text-book account.
NOTE 3.-The recent observations of G. Fineman, Anat. Hefte, 159 H. (53 B. H.), 1915, show that the ductus endolymphaticus is not derived from the original canal of communication with the exterior, but is formed independently by a process of evagination.
NOTE 4.-Evidence which has accumulated since this statement was made tends to show that blood orpuscles and the endothelial cells which form the walls of the primitive blood-vessels are derived from ferent ancestors, the endothelial cells from mesenchyme cells, and the red blood corpuscles form angioblasts which be derived, as some observers believe, from mesenchyme cells, or, as others think more probable, from entoderm cells.
NOTE 5.-The origin of the white blood corpuscles is still uncertain; according to some investigators they and the red corpuscles have common ancestors and the same ancestors may produce endothelium also; this s the so-called monophyletic view. It appears probable, however, that, in some vertebrates, the white orpuscles are derived from one set of mesoderm cells, the red corpuscles from another, and the endothelium of the blood-vessels from a third set of mesodermal cells, each set of mesoderm cells being capable of provacing only one kind of descendant; this is the polyphyletic view.
when the umbilical orifice closes. The extra-embryonic portion is entirely obliterated when the outer surface of the expanding amnion fuses with the inner surface of the chorion (compare Figs. 77 and 78).
The Intra-embryonic Colom.-The intra-embryonic cœlom appears as a series of cleft-like spaces in the margin of the embryonic mesoderm. The spaces fuse together to form a n-shaped cavity (Fig. 89) which separates the peripheral part of the embryonic mesoderm into a parietal or somatic, and a visceral or splanchnic, layer. The bend of the n-shaped cavity lies in the margin of the cephalic part of the embryonic region, and it has no direct communication with the extraembryonic cœlom, but the greater part of each stem of the cavity, on account of the disappearance of its lateral wall, soon opens, laterally, into the extraembryonic cœlom.
The transverse portion of the n-shaped cavity, which extends across the cephalic end of the embryonic area and connects the two limbs together, is the pericardial cavity. The adjacent part of each lateral limb of the cavity is the pleuro-pericardial canal, it becomes a pleural cavity, and the remaining portions of the two limbs FIG. 89. SCHEMA OF INTRA- unite ventrally, as the umbilical orifice closes, to form EMBRYONIC COLOM SEEN FROM the single peritoneal cavity.
ABOVE BEFORE THE FOLDING OF
As the head fold forms, the pericardial part of the cavity is carried ventrally and caudally into the ventral wall of the fore-gut (Fig. 90). The mesoderm which originally formed its peripheral boundary, but which now lies in the cephalic boundary of the umbilical orifice, becomes thickened, and forms the septum transversum (Figs. 90, 91, 93).
BUT BEFORE THE
FIG. 90.-SCHEMATA OF EMBRYONIC COLOM AFTER FOLDING OF EMBRYONIC AREA
At the cephalic end of its dorsal wall, on each side, the pericardial cavity is still continuous with the two lateral parts of the cœlom; and each lateral part, which
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
Opening into pericardium
Colom.-In the lateral wall of each
FIG. 91.-SCHEMA OF LATER STAGE OF DIFFERENTIATION
OF COLOM. A, from above. B, transverse section cut FIG. 92. SCHEMA OF A TRANSVERSE SECTION
pericardial canal (Fig. 91). As the lung buds grow the cavities of the pleuro-
pleura and pericardium
Duct of Cuvier
the pericardium towards the ven-
Closed aperture between the cavity of each pleuro-pericardial
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 side
of the trachea and the oesophagus, until its cavity is obliterated. When this occurs the pericardial cavity is entirely shut off from the remainder of the cœlom, 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 cœlom surrounding the lung, the original pleuro-pericardial canal, is separated from the more caudal part of the cœlom, 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 24 by 18 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 cœlom 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