CHAPTER IV. 

 STUDY OF PIG EMBRYOS. 



Methods of Obtaining Embryos. 



The pig is recommended for embryological study because specimens of the 

 embryos in sufficiently early stages can be obtained at the larger packing estab- 

 lishments in considerable numbers and with little trouble or expense. When 

 this material is not obtainable, rabbit embryos may be substituted, as these 

 animals are easily kept and breed freely (compare page 305). Owing to the enor- 

 mous precocious development of the chorionic vesicle in pigs, it produces an en- 

 largement of the uterus which is usually sufficient, by the time the embryo has 

 attained a length of 6 mm., to be observable to the untrained eye. It is, there- 

 fore, only necessary to ask the man who removes the viscera from the pigs to lay 

 aside for examination all of the uteri which appear distended. These should not 

 be turned about violently, but handled carefully and should be opened immedi- 

 ately. As soon as the ovum is exposed it will probably be ruptured, and there 

 will occur a free outflow of opalescent fluid, amniotic and allantoic. With the 

 aid of fine forceps and a horn spoon the embryo may be lifted up — and it should 

 on no account be directly touched — and transferred to a dish containing Miiller's 

 fluid, in which the specimen should remain for five or ten minutes. It is then 

 transferred with the help of the horn spoon to Zenker's fluid. Metal instruments 

 cannot be used on account of the corrosive sublimate in the Zenker solution. In 

 one or two hours the embryo should be transferred to fresh Zenker solution and 

 left therein a varying length of time, according to the size of the specimen. In 

 general it may be said that for— 



Pigs of 6 to 9 mm., 12 hours. 



" " 12 " . 24 " 



" " 15 " • • 36 " 



'• " 20 to 25 " 48 " 



It is undesirable to leave any specimen in the Zenker solution more than 

 forty-eight hours. The Miiller's fluid is used for cleaning the specimen. It 



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