Of the internal organs of the embryo, the stomach is now a more con spicuous dilatation than before; while the small intestine has elongated so as to form a loop, from the apex of which the slender yolk-stalk still arises; lower down, at the junction of small and large intestines, a rudi ment of the cecum has appeared. The lungs, liver, and heart have all increased in size, while the right umbilical vein, which has been all along the smaller of the two veins returning blood from the allantois to the embryo, has disappeared. The Wolffian bodies are rather shorter than before, and along their inner borders two slight thickenings of the peri toneal epithelium—the genital ridges—have appeared. There are also present two new ducts, the Mfillerian ducts, which will become in the female child the oviducts or Fallopian tubes, and the uterus and vagina.
Sixth Week.
The embryo nas grown considerably, and is now from .6 to .8 inches in length. It is shown in situ in Fig. 119; while Fig. 120 is a profile view on a larger scale.
Fig. 119 shows us that the amnion is now a considerable distance from the embryo, and has indeed nearly reached the chorion; that the allantoic stalk is still short and thick; that the yolk-stalk is long and slender, its proximal portion being bound up with the allantoic stalk in the sheath formed round both by the amnion (cf. Fig. 98, 5), while its distal portion, ending in the small yolk-sac, lies betweenthe amnion and chorion (cf. Figs. 98 and 124). The limbs are larger, and show at their extremities rudi ments of the fingers and toes. The mouth is still very wide; a slight prominence above it marks the commencement of the nose, and the mar gin of the hyomandibular cleft forms a slightly projecting ring, the ex ternal ear.
Of the internal organs, the alimentary canal has increased in length, and the mecum is now very evident; the Wolf an bodies are somewhat smaller than before, while the genital organs have increased in size, and the per manent kidneys and ureters have become definitely established. Finally, by the end of the sixth or commencement of the seventh week, ossification commences in the clavicle, and very shortly afterwards in the lower jaw.
Second Month.
At the end of the second month the embryo measures from 1.1 to 1.6 inches in length, and weighs 180 to 300 grains. The head is very large, _ and forms at least a third of the whole embryo. The nose is rather more prominent, but is still very small; slight folds of skin round the mouth and eyes mark the commencement of the lips and eyelids. The external
ear is definitely established, and in it helix and antihelix, tragus, and antitmgus, can already be recognized. The limbs project some little distance from the trunk; the bend of the elbow being directed backwards, and that of the knee forwards. The ventral wall of the abdomen is com pletely formed; and the umbilical cord, which usually measures about .39 inches in length, is as a rule straight, but may be slightly twisted on itself. The anus is marked by a dark point, and the rudiments of the external organs of generation are visible; ossification has commenced in the frontal bones-and in the ribs and in many of the bones of the limbs. The epi dermis can now be distinguished from the dermis.
. Tenth Week.
The embryo has now a length of 1.5 to 2 inches, and weighs 675 to 720 grit.ins. The limbs are still short, but their several divisions are far more evident than before, and rudiments of the nails have commenced to appear as small tubercles.
The ventral wall of the body is far more completely formed than here tofore. The umbilical stalk, formed, it will be remembered, of allantoic stalk and yolk-stalk bound together and ensheathed by the amnion, has grown considerably: it is now longer than the embryo, and is twisted on itself in a spiral manner. It still contains at its base a loop of intestine.
The face has developed considerably, and all the features are now defi nitely established; the eyelids are present; there is a distinct though very flat nose, definite lips, and well-developed external ears. In Fig. 121 the leading stages in the development of the face are shown. At the sixth week, B, the mouth opening is still very wide; it is bounded in front by the median fronto-nasal process, at the sides by the maxillary arches, and be hind by the mandibular arches, which meet one another in the middle line at the site of the future chin. By the eighth or ninth week, C, the maxillary processes have grown in towards one another so as to reduce the width of the mouth, and have fused with the fronto-nasal process to com plete the upper jaw. The nose is an outgrowth from the fronto-nasal pro cess; folds of integument give rise to the eyelids and lips, while another fold, arising behind the hyomandibular cleft, forms the external ear.