The whole of the large intestine is filled with a dark olive-green viscous meconium. The liver is still very large relatively to the whole body, and is of a deep brownish-red color The testes have, as a rule, descended as far as the inguinal ring, and may even have entered the inguinal canal.
The end of the seventh month is of interest as being perhaps the earliest period at which the foetus can be born with any reasonable chance of sur viving.
Eighth Month.
During the eighth month the increase in bulk is far more marked than that in length. At the end of the month the foetus measures from 15 to 17 inches in length, and weighs as much as 41 to 5 pounds.
The skin is of a brighter flesh color than before, and is covered all over with the sebaceous deposit known as " vernix caseosa." This substance, which usually makes its first appearance about the middle of gestation, was formerly considered to be a deposit formed from the liquor amnii, but appears rather to consist of mattet formed by the cutaneous glands of the foetus, mixed with dead epithelium cells. It varies much in quan tity in different cases, and is always more abundant in certain situations, notably the head, axillte, and groins.
The chin is now far more prominent than before, the lower jaw equal ing the upper in length. The pupillary membranes, if over present, are at any rate absent now. One of the testes, usually the left one, has passed through the inguinal canal into the scrotum, while the other one is still in the canal as a rule. There is no ossification in the lower epiphy sis of the femur.
Ninth Month.
At the full time the foetus is 19 to 23 inches long, and weighs on the average 61 to 7 pounds.
The skin is paler than 'before. The cellular tissue is filled with fat, giving roundness and firmness to the body and limbs. The hair is thick, long, fairly abundant as a rule on the head, while the down has begun to disappear from the body.
The umbilicus, formerly supposed to mark the exact middle of the body at full term, is stated by Cazeaux, on the authority of Moreau and 011ivier, to be on the average as much as .8 inches below the middle point.
Both testes are as a rule in the scrotum, which has now a-corrugated surface.
Ossification has commenced in the centre of the cartilage at the lower end of the femur. This is the first epiphyseal ossification to appear in the body, and is the only one present at the end of the ninth month. Its
presence appears to be very constant at this period, and it has therefore received much attention as a ready and apparently reliable test of a fetus having reached its full time. • The Fetal Membranes.
The youngest stage in the development of the human ovum that has yet been found in the uterus is, as we have seen, that described by Reichert, and figured on page 174. This ovum, estimated to be thirteen days old, • was already completely invested in a decidua reflexa. Its outer wall was described by Reichert as consisting of a single layer of epithelial cells, a description accepted also by His; but we have seen above that there is hardly any doubt that the wall is not of so simple a structure, but that immediately under the epithelial layer there is an inner vascular layer. As the vessels in this layer can be traced at a rather later stage into continuity with the umbilical vessels of the foetus, there is little room for doubt that this inner vascular layer is really the allantois, developed very early relatively to the other organs, and in a very unusual manner.
The C'horion.
Such being the case, we may speak of the outer wall of Reichert's ovum as a chorion. A typical chorion, as we have seen, consists of three originally separate and distinct membranes fused together to form a single one—(1) on the outside the vitelline membrane, or zona pellucida; (2) within this the subzonal membrane, or false amnion; (3) within this again, the allantois. In the chorion of the early human ovum the zona pellucida does not appear to be recognizable; the epithelial layer may possibly be in part the equivalent of the subzonal membrane; while the inner vascular layer is almost certainly the allantois.
Reichert's ovum is surrounded by a broad marginal zone of villi, the centres of the two flattened surfaces forming bare patches. A short time later, towards the end of the third week, the villi extend so as completely t) surround the ovum; they consist at first merely of epithelial cells de rived from the outer layer of the chorion; but in the course of the fourth week, according to Coste, outgrowths from the vascular layer of the chorion enter the villi, each of which now consists of an external epithelial covering and a central connective-tissue vascular core, the vessels of which are continuous with the umbilical vessels of the embryo.