CHAPTER VI. 

 STUDY OF PIG EMBRYOS. 



Method 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 ani- 

 mals are easily kept and breed 'freely (compare page 166). The enormous pre- 

 cocious development of the chorionic vesicle in pigs produces an enlargement 

 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, therefore, 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 immediately. 

 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 Muller'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 for 



Pigs of 6 to 9 mm 12 hours. 



Pigs of 12 mm. ..:... - . 24 hours. 



Pigs of 15 mm 36 hours. 



Pigs of 20 to 25 mm 48 hours. 



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

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

 a granular, non-adherent coagulum to form from the fetal fluids. If the speci- 

 men is put directly into Zenker's fluid, a fibrous coagulum is formed which often 

 adheres closely to the embryo so as to obscure its shape. Such a fibrous co- 



219 



