562 EMBRYOLOGY. 



^ inch ; at 24 days, i to f inch. The umbilical vesicle has 

 reached its maximum size. 



(e.) Carnivora. — The villi appear on the chorion, and become 

 attached to the mucous membrane of the uterus. At 18 

 days, the embryo of the dog has a length of about i inch ; the 

 primitive groove and chorda dorsalis are present. At 19 or 20 

 days, the heart is present as an S-shaped tube ; the intestine 

 beains to form. At 21 days, indication of the eyes, ears, and 

 Wolffian body are present ; the limbs are present as small 

 blunt processes. 



III. Period. — Includes, in the horse and ox, the 5 th to the 

 8th week ; in the sheep and goat, the 5th, 6th, and first half of 

 the 7th week ; in the pig, the last half of the 4th, the 5th, and 

 first half of the 6th week ; in carnivora, the 4th week. 



(a.) Horse. — The horse's embryo, at 6 weeks, has a length of 

 If inch ; at 7 weeks, 2 inches ; and at 8 weeks, 2f inches ; the 

 thoracic and abdominal cavities are closed. 



(5.) Qx. — At 6 weeks, the embryo is f inch in length, and 

 corresponds to the 22-23 days-old dog's embryo. At 7 weeks, 

 it is about li inch in length ; the udder begins to form ; the 

 kidneys and ureters are present. At 8 weeks, the embryo is from 

 2 inches to 2f in length. 



(c.) Sheej). — At the 5th week, the embryo has a length of 

 1 inch, and, in development, corresponds to the 25 days-old 

 dog's embryo. 



(d.) Pig. — At 28 days, the embryo has a length of 1 inch. 

 At 87 days, 2i inches. 



(e.) Carnivora. — At 22-23 days, the embryo has a length of 

 about f inch ; mouth-groove, nostrils, lungs, trachea, and larynx 

 are present ; the anus is 'still closed ; Wolffian body well formed ; 

 limbs about one line in length. At 25 days, the thorax is closed, 

 and the abdominal cavity almost closed. At 26-28 days, the 

 embryo is about If inch in length ; all organs are more or 

 less distinctly marked, 



IV. Period. — Includes, in the horse, the 9th to the 13th 

 week; in cattle, the 9th to the 12th week; in the sheep and 

 goat, the last half of the 7th, the 8th, and the 9th weeks ; in 

 pigs, the last half of the 6th, the 7th, and first half of the Sth 

 week ; in carnivora, the 5th week. 



