The original opening of the diverticulum into the cesophagus becomes modified to form the glottis, while the median portion of the diverticulum lengthens and becomes the trachea.
Fourth Month.
At the end of the fourth month the fcetus measures from 6 to 7 inches in length, and weighs from 3450 to 3900 grains. The skin is of a rosy color, and is much firmer than before. Short whitish hairs appear on the head, and a slight down on other parts of the body. The eyes, nostrils, and mouth are all closed. The chin, which has hitherto been very incon spicuous, begins to become prominent. The legs and arms are of about equal length. The external sexual characters are usually well marked. The anus is open, and the duodenum contains meconium of a light grayish-white color. The umbilicus, or point of origin of the umbilical cord, is low down, close to the pubes. In the skull the bones are still far from meeting one another, so that the sutures and fontanelles are very wide. The muscles are more fully developed, and may give rise to dis tinct movements of the fcetus. In abortions at this period the foetus may live for some hours.
Fifth Month.
Length of body, 8 to 10 inches; weight, 3750 to 4500 grains. From this time onwards, according to Casper, the length of the fcetus affords a ready and easily remembered means of determining roughly its age; for from the fifth month to the end of pregnancy the length of the body in inches is approximately double the number of lunar months the foetus has lived. Thus, at the fifth month the length is 10 inches, at the sixth month 12 inches, and so on. The weight is subject to far greater varia tions than the length, and consequently affords a far less trustworthy criterion of age.
The skin is more consistent than before, and presents on its surface at certain places small patches of sebaceous matter. Hairs are more exten sively developed, but are still devoid of any distinct color. The legs are now longer than the arms, and the nails are well formed. The umbilicus is further forward than at the preceding month, and is now some distance above the pubes.
The head is still very large in proportion to the other parts. The
heart, liver, and kidneys are also disproportionately large. The small intestine contains meconium, which is now, owing to the secretion of bile, of a pale greenish-yellow color. The gall-bladder is distinct.
Ossification has commenced in the pubes and in the os calcis.
Sixth Month.
The average length, according to Casper's calculation, is 12 inches. The weight is much more variable; its average amount is stated by Cazeaux to be pounds.
The skin is of a dirty reddish color and much wrinkled. It is covered, at any rate in the axillie and groins, with a sebaceous deposit. The hairs are more strongly developed and of a darker color than before. Both eyelashes and eyebrows have commenced to appear.
A complete pupillary membrane is commonly said to be present; but there seems to be some doubt on this point, and according to Velpeau and Cazeuux a large pupillary aperture is already present in the iris. In deed, Velpeau contends that no pupillary membrane is ever present in the human embryo.
The umbilicus is still further forward than before. The meconium is mud' darker and more viscous than before. The testes of the male have not yet descended into the scrotum, but are found within the abdominal cavity, lying on the psoas muscles and immediately below the kidneys.
The sternum is well developed and has commenced to ossify. The nails reach to the ends of the fingers, and extend about a quarter of the way round them.
Seronth Month.
At the end of the seventh month the foetus has a length of from 12 to 14 inches, and weighs, on an average, about 21- pounds.
The skin is still of a dirty reddish color, but is not so 'dark as it has hitherto been. There is an increased deposit of fat in the cellular tissue, causing the body to appear more plump and round. The hairs are plen tiful and about a quarter of an inch in length.
The several bones forming the roof of the skull become strongly con vex, the central portion of each, whence ossification starts, forming a very evident prominence. The eyelids, which have been closed since reaching their size in the fourth month, now open.