through its thickness and are attached to the chorion. In the early months these septa convey branches of the uterine arteries which open into the intervillous space on the surfaces of the septa. The fetal surface of the placenta is smooth, being closely invested by the amnion. Seen through the latter, the chorion presents a mottled appearance, consisting of gray, purple, or yellowish areas. The umbilical cord is usually attached near the center of the placenta, but may be inserted anywhere between the center and the margin; in some cases it is inserted into the membranes, i. e., the velamentous insertion. From the attachment of the cord the larger branches of the umbilical vessels radiate under the amnion, the veins being deeper and larger than the arteries. The remains of the vitelline duct and yolk-sac may be sometimes observed beneath the amnion, close to the cord, the former as an attenuated thread, the latter as a minute sac.
On section, the placenta presents a soft, spongy appearance, caused by the greatly branched villi; surrounding them is a varying amount of maternal blood giving the dark red color to the placenta. Many of the larger villi extend from the chorionic to the decidual surface, while others are attached to the septa which separate the cotyledons; but the great majority of the villi hang free in the intervillous space.
An image should appear at this position in the text.Fig. 40.—Embryo between eighteen and twenty-one days. (His.) An image should appear at this position in the text.Fig. 41.—Head end of human embryo, about the end of the fourth week. (From model by Peter.)
THE BRANCHIAL REGION.
The Branchial or Visceral Arches and Pharyngeal Pouches.—In the lateral walls of the anterior part of the fore-gut five pharyngeal pouches appear (Fig. 42); each of the upper four pouches is prolonged into a dorsal and a ventral diverticulum. Over these pouches corresponding indentations of the ectoderm occur, forming what are known as the branchial or outer pharyngeal grooves. The intervening mesoderm is pressed aside and the ectoderm comes for a time into contact with the entodermal lining of the fore-gut, and the two layers unite along the floors of the grooves to form thin closing membranes between the fore-gut and the exterior. Later the mesoderm again penetrates between the entoderm and the ectoderm. In gill-bearing animals the closing membranes disappear, and the grooves become
