DEVLLOFMENT OF THE FCETUS. 561 



thorn is returned to the placenta to be purified. At birth, when 

 respiration. ha,s been established, the placental circulation is cut 

 off, the foramen ovale rapidly closes up, leaving the fossa 'ovalis ; 

 the ductus arteriosus contracts, and together with the umbilical 

 arteries and vein, becomes obliterated 



The consideration of these facts will show that the foetus is 

 nourished by blood less perfectly arterialised than that of the 

 adult, the blood becoming mixed in the liver, in the heart, and 

 in the posterior aorta by means of the ductus arteriosus. It 

 should be borne in mind that the blood which supplies the 

 posterior regien is less pure than that supplying the bead end 

 anterior region ; this accounts for the fact that at birth the 

 posterior parts of the frame are less developed than the anterior. 



Development of the Fcetus. 



The following facts and measurements derived from the 

 observations of Gurlt, Leyh, Franck, and others, may be useful 

 in determining the age of the fcetus or embryo. 



To facilitate the study of the development of the foetus, the 

 time which elapses between conception and birth has been divided 

 into seven periods as follows : — 



I. Period. — Includes, in all the domesticated mammals, the 

 first and second weeks after conception. The ovum lies free in 

 the uterus. In the dog it measures about one line in length. 

 The chorion and amnion can be made out. 



II. Period. — Includes, in the horse and ruminant, the 3rd 

 and 4th weeks ; in the pig, the 3rd and half of the 4th week ; 

 in carnivora, the 3rd week. 



(a) Horse.— At 28 days the embryo is a little over J inch 

 long. The division into head, trunk, and limbs is indicated. 



(6.) Ox. — At 28 days, the embryo has a length of f inch, 

 and corresponds in its development to the sheep's embryo of 25 



(c.) Sheep. — Theembryo of the sheep, at 18 days, has a length 

 of -^ inch. At 23 days, indications of the feet appear. At 25, 

 days, it has a length of ^ inch ; indications of the eyes, ears, and 

 tail are present ; the intestine exists as a tube connected with 

 the umbilical vesicle ; the Wolffian body is well developed. 



(d.) Pig. — The pig's embryo, at 21 days, has a length of about 

 ^ inch; at 24 days, -| to f incL The umbilical vesicle has, 

 reached its maximum size. 



