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 oesophagus, 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 Uh day. — Owing to the still further 

 diminution of the white of the egg, the embryo lies 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 stalk 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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