EMBRYOLOGY 209 
Cranial and spinal nerves appear as lateral outgrowths of the 
central nervous system. 
The foregut and the hindgut are completed; the former is 
divided into cesophagus, stomach, and duodenum, the hindgut into 
large intestine and cloaca. 
The formation of the lungs from a ventral diverticulum of the 
alimentary canal immediately in front of the stomach. 
The diverticular outgrowths from the duodenum form the liver 
and the pancreas, the ducts of these glands being the lengthened 
stalks of the outgrowths. 
A pair of primitive excretory organs appears in the proximal 
corners of the walls of the pleuroperitoneal-cavity, as the 
“ Wolffian” ducts and bodies. 
The embryo itself has turned over so that it now lies on its left 
side. 
Changes during the 4th day.—Owing to the still further 
diminution of the white of the egg, the embryo les almost in im- 
mediate contact with the shell membrane. ‘The vascular area is 
about as large as a halfpenny, and the whole blastoderm embraces 
more than half of the yolk. The amnion completely encloses the 
embryo, which by this time has been so much folded-off from the 
yolk-sac, that the connecting stalk is much constricted. The inner 
or splanchnic stall is now called the vitelline duct. 
The head of the embryo is bent ventralwards at more than a 
right angle, forming the cranial flexure. The tail is curved inwards 
and forms a conspicuous feature, the whole embryo being somewhat 
spirally curled up on itself. 
The anterior and posterior extremities make their appearance 
as flattened conical buds. 
The cerebral hemispheres and the optic vesicles have enormously 
increased in size. 
The nose, ears, and jaws become more distinct. The ovary, 
kidneys, and ureters are formed. The allantois projects as a small 
pear-shaped bag and receives allantoic vessels from the vitelline 
veins and from the dorsal aorta. 
Changes during the 5th day.— 
The blastoderm has spread over the whole of the yolk-sac, and 
the yolk is thus completely enclosed in a bag, whose walls, however, 
are excessively delicate and easily torn. ‘The vascular area extends 
over about two-thirds of the yolk. The splanchnic stalk or vitelline 
duct has been reduced to a narrow solid cord. The allantois 
serves already as the chief organ of respiration, and stretches far 
over the right side of the embryo in the cavity between the two 
amniotic layers. The embryo, lying on its left side, remains ex- 
tremely curved, so much that the head and tail are nearly in 
contact. The fore- and hindlimbs have become lengthened, elbow 
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