The third aortic arch (Fig. 115, 3) forms the internal carotid artery, but is still connected with the dorsal aorta. The fourth arch (Fig. 115, 4) forms the main portion, and later on the whole of the dorsal aorta; the arches of the two sides, right and left, are at this stage equal in size. The fifth arch (Fig. 115, 5) joins the dorsal aorta, but before doing so gives off a branch—the pulmonary artery—to the lung of its side.
The dorsal aorta, formed in this way by the third, fourth, and fifth aortic arches of each side, runs down the dorsal surface of the embryo, giving off small arteries to the alimentary canal and Wolffian bodies. About the level of the cloaca it divides into the two umbilical arteries which convey the blood of the embryo along the allantoic stalk to the chorion, whence it returns charged with nutrient matter by the umbilical veins, and so passes by the inferior vena cava to the right auricle.
The vessels in connection with the yolk-sac are still present, but are of no great importance. The vein returning blood from the yolk-sac, after receiving the veins from the alimentary canal and thereby forming the portal vein, enters the liver, and there unites with the right umbilical vein coming from the chorion.
• All the vessels are at present merely tubular channels in the tissue of the embryo, lined by endothelium, but with no proper connective tissue or muscular walls of their own.
Concerning the other organs of an embryo of this age, we may notice the following points: The Wolffian bodies or primitive kidneys (Fig. 115, L'n) extend forwards as far as the lungs. Each consists of a series of short convoluted tubes, commencing with dilated Malpighian bodies, and opening at their other ends into the Wolffian duct. The two Wolffian ducts open separately into the cloaca, nearly opposite to the opening of the allantois. Of the permanent kidneys and ureters there is no trace, unless a small saccular dilatation on each Wolffian duct close to its open ing into the cloaca be the commencement of the ureter (Fig. 115, U).
The large size of the head, due almost entirely to the brain, is a strik ing feature in nearly all vertebrate embryos of this age, though less marked in mammals than in some of the lower forms. In the human embryo the brain at this stage dorms about one-third the total length of the em bryo. Besides all the main divisions of the brain, the Gasserian ganglion on the fifth nerve, and the ganglionic swellings in the auditory, glosso pharyngeal, and pneumogastric nerves are very evident (Fig. 115).
The olfactory organs are present as shallow pits on the under surface of the head, communicating by grooves—the rudiments of the posterior nares—with the mouth. The eyes are of very small size as compared
with other vertebrate embryos, and the lens is still in the form of an open pit. The auditory organs (Figs. 113 and 115) are closed pear-shaped vesi cles, situated just above the tops of the hyomandibular clefts—the rudi ments of the future tympano-Eustachian passages—but having as yet no connection with them; rudiments of the aqueductus vestibuli, semicircular canals, and cochlea are present as slight outgrowths from each vesicle.
Finally, we may direct attention to the presence of a very rudimentary diaphragm between the heart and liver, and to the short, stumpy tail, in which the number of vertebra) is never greater than that present in the coccyx of the adult.
• Fifth Week.
The rudiments of all the more important organs being already definitely established, there will be no occasion to describe the later embryos in detail; but it will suffice to state the leading features of each of the well marked periods of development, and to give brief accounts of the forma tion of some of the more important organs up to the time of birth.
At the end of the fifth week the embryo measures about .39 inches in length, and weighs about 15 grains. Its external appearance is shown in Fig. 116, and the leading features of its anatomy in Fig. 115.
In this latter figure the embryo is represented from the ventral side, and is partially dissected, the liver being completely removed. The chief features in which it differs from thp embryo of the fourth week are the following: The whole body is not nearly so strongly flexed, but has begun to straighten out. The limbs are considerably larg3r, and already show signs of division into their several segments. The gill clefts, with the exception of the hyomandibular cleft, have completely closed up, and the face is more fully formed than before. The yolk-sac is small, and is connected with the embryo by a long slender stalk. The allantoic stalk is still short and thick. The amnion, instead of closely investing the embryo, is now at some distance from it, and is very nearly in contact with the chorion. The villi of the chorion are very large and branched, and still extend over the whole surface of the ovum, though they are rather larger opposite the spot where the ovum is directly attached to the uterus—i.e., the decidua scrotina, the site of the future placenta—than they are elhewhere.