FCETAL ENVELOPES. 517 



vesicle. This disappears as the period of gestation draws 

 to a close. 



Placenta. — It is by means of the placenta that the fcetu 

 gets its supply of pure blood. Here it is that the inter- 

 change of gases takes place by the process of osmosis ; the 

 blood of the foetus receiving oxygen from that of the 

 mother, and at the same time giving out its carbon dioxide. 

 The placenta in the mare is attached by little tufts or villi 

 to the uterus ; while in the cow the attachments are known 

 as cotyledons, and vary in number from sixty to seventy. 

 Eetention of the placenta in the mare is very rare indeed 

 as compared with its frequency in the cow. The umbilical 

 cord contains two umbilical arteries and one vein imbedded 

 in a mass of gelatinous material. The urachus passes from 

 the anterior part of the bladder and terminates in the 

 alantoid cavity. Occasionally a case is met Avith of a foal 

 two or three days old with the urine dribbling from the 

 umbilical opening, showing that the urachus has remained 

 pervious instead of withering up, as it usually does, forming 

 a lisjament for the bladder. The treatment consists of 

 closing the opening by scarifying, suturing, etc. 



GESTATION. 



The period of gestation in the mare is usually eleven 

 months, but it may vary to a remarkable extent, in some 

 mares being only ten months, whilst in others thirteen 

 months may elapse from the time of conception up to 

 the time the young animal is born. ^lale foals are carried 

 two or three days longer than female foals ; and, as a rule, 

 an old mare carries her foal longer than a young mare. The 

 period of pregnancy with the cow is also subject to great 

 variation, but on an average is nine months and a-half. 

 The sheep and goat carry their young about five months ; 

 the sow four months ; the bitch two months, or sixty- 



