GENERAL ANATOMY OF EMBRYO OF 12 MM. 231 



of the right ventricle by means of the interventricular foramen, For.iv. Above 

 the ventricle is seen the main trunk of the aorta, which divides and gives rise to 

 three aortic arches, Ao.III, Ao.IV, Ao.V. The first and second arches are still 

 present, but are very slender. Their common stem, Car.ex, forms the future stem 

 of the external carotid. From the dorsal end of the third arch, Ao.III, there runs 

 forward the vessel, Car.in, which joins the first and second arches, and itself be- 

 comes part of the stem of the internal carotid. The aortic arches form on each 

 side of the descending aorta, Ao.D, which unites with its fellow just above the level 

 of the stomach to form the main median dorsal aorta, Ao. The aorta gives, off a 

 series of intersegmental vessels which mount dorsalward between the spinal cord 

 and the outer surface of the embryo. The main aorta gives off on its ventral side 

 a vessel, Cce.ax, which becomes the cceliac axis of the adult; and lower down, three 

 vessels, A.o.m, which anastomose in several places. In their course to the yolk-sac 

 they cross to the right of the intestine and give off branches to the mesentery. The 

 two upper roots atrophy later, but the lower persists to form the stem of the 

 superior mesenteric artery. Just before reaching the cloaca the aorta gives off 

 vessels, A.il } which run to the hind leg, and develop into the external iliac of 

 the adult. Just beyond, the aorta gives off a small branch, A.caud, to the tail; and 

 at the same time bifurcates to form the umbilical arteries, A.um, which take a 

 sinuous course alongside the allantois, All, and ultimately ramify outside of the 

 body of the embryo in the walls of the allantois. The allantois itself springs from 

 the cloaca as a narrow canal which makes a sharp bend and runs to the umbilicus, 

 gradually expanding. Outside of the embryo, the allantois forms; in the pig as 

 in all ungulates a very large vesicle. In man and the primates, on the contrary, 

 the allantois is rudimentary. This is one of the most striking differences between 

 the pig and the human embryo. 



Of the veins little is shown in this figure, but the omphalo-mesaraic and portal 

 veins are included. The former, V.om, arises on' the surface of the yolk-sac, 

 passes through the umbilicus into the mesentery of the intestine, and extends 

 parallel with the ileum until it crosses it, a little posterior to the pancreas. It is 

 there joined by another vein, V.s.m. the superior mesenteric, which is of about 

 the same caliber. The common trunk formed by the union of these two veins is 

 the portal vein, V .port, which divides into two branches, extending to the right and 

 left of the omphalo-mesaraic vessels. The left branch is small. It passes on the 

 left of the dorsal pancreas and extends to the liver. The right branch passes 

 between the two pancreatic anlages, then bends to the right and enters the liver, 

 into the sinusoids of which it discharges its blood-stream. 



Pig Embryo of 12.0 mm. General Anatomy. 



Dissection of the Viscera (Fig. 170). The specimen figured measured 12.7 

 mm. Both limbs and the body-wall on the left side have been removed, displaying 

 the organs in situ. The umbilical cord has been cut lengthwise so as to display its 



