Enbryology / All About Embryology

iDuring the third month the young fetus clearly resembles a human being, although the head is disproportionately large (Figure 1P). The previous protrusion of much of the intestine into the umbilical cord is reduced through the return of its loops into the abdomen. The ears rise to eye level and the eyelids fuse shut. Nails begin forming; ossification (bone-forming) centres appear in most of the future bones; and the sex of external genitalia becomes recognizable. (In this paragraph, and in the next two, the months are lunar months, of 28 days).

At four months individual differences between the faces of fetuses become distinguishable. The face is broad but the eyes are now less widely separated. The umbilical cord attaches higher on the abdominal wall; this location is above an expanding region between the cord and the pubis (front bones of the pelvis) that scarcely existed previously.

At five months downy hairs (lanugo) cover the body, and some head hairs appear. The skin is less transparent. Fetal movements (“quickening”) are felt by the mother. At six months eyebrows and eyelashes are clearly present. The body is lean, but its proportions have improved. The skin is wrinkled. At seven months the fetus resembles a dried-up old person. Its reddish, wrinkled skin is smeared with a greasy substance (vernix caseosa). The eyelids reopen. At eight months fat is depositing beneath the skin. The testes begin to invade the scrotum. At nine months the dull redness of the skin fades and wrinkles smooth out. The body and limbs become better rounded.

At full term (38 weeks) the body is plump and proportions are improved, although the head is large and the lower limbs are still slightly shorter than the upper limbs. The skin has lost its coat of lanugo hair, but it is still smeared with vernix caseosa. Nails project beyond the finger tips and to the tips of toes. The umbilical cord now attaches to the centre of the abdomen. The testes of males are usually in the scrotum; the greater lips of the female external genitalia, which previously gaped, are now in contact. Cranial bones meet except at some angular junctions, or “soft spots.”

The average time of delivery is 280 days from the beginning of the last menstrual period, whereas the duration of pregnancy (age of the baby) is about 266 days (38 weeks). Pregnancy may extend to 300 days, or even more, in which case the baby tends to be heavier. Even when treated in a neonatal unit, premature babies born under 27 weeks of age are less likely to survive, whereas those more than 30 weeks old usually do survive.

Abnormal development

Multiple births

It is both unusual and abnormal for the human species to produce more than one offspring at a time. “Twins” and “twinning” are used as general terms for any number of multiple births, as the same basic principles apply.

Fraternal twins stem from multiple ovulations in the same cycle. Each oocyte develops singly in a separate follicle, is shed and fertilized individually, develops within its own chorionic sac, and forms an individual placenta. In some instances, two blastocysts implant close together and the expanding placentas meet and fuse. In such double placentas, however, the two blood circulations rarely communicate. The word dizygotic technically designates two-egg twins. Such pairs obviously are independent in sex determination and bear no more resemblance than do other children of the same parents. Properly speaking, they are merely littermates. Nearly three-fourths of all American twins are dizygotic, whereas the Japanese ratio is only one-fourth. A tendency toward such multiple births exists in some family lines.

Wholly different are those true twins who are always of the same sex and are strikingly similar in physical, functional, and mental traits. Such close identity is enforced by their derivation from a single ovulated and fertilized egg, and hence by their acquisition of identical chromosomal constitutions. This twin type is named monozygotic. Three-fourths of such pairs develop within a common chorionic sac and share a placenta; one-fourth have individual sacs and placentas. The latter condition results from a mishap before implantation, when the cleavage cells separate into two groups and then become individually implanting blastocysts. There is no discernible hereditary tendency toward the production of monozygotic twins.

Several atypical processes involving the inner cell mass or embryonic plate can produce separate embryos within a single sac: (1) The inner cells of a blastocyst may segregate into two masses. (2) Somewhat later in time, two embryonic axes may become established on a single embryonic disk. (3) A single axis may subdivide by fission or budding. (4) Duplication of any sort may combine with secondary subdivision; the Dionne quintuplets are believed to have followed this sequence, which is also normal for the regular quadruplets of the Texas armadillo.

Occasionally monozygotic twinning can result in fused, or conjoined, twins. Conjoining results from divergent growth at the front or hind end of the emerging primitive axis of an embryo, or at both ends. The degree of union varies from slight to extensive, and the possession of a single or double set of internal organs depends on the intimacy of fusion at any particular level. Union occurs by the heads, upper trunks, or lower trunks; the joining may be by the dorsal, lateral, or ventral surfaces. Sometimes there is a marked disparity in the size of the two twins; this condition is known as nonsymmetric twinning, and usually the much smaller twin will be dependent on the larger for nutrition.

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