122 WINIFRED HUGHES. 



In the pig, the two apices of the chorionic vesicle do not enter into 

 formation of the placenta. The chorion is thin in these regions, 

 being stretched by the extension of the allantois inside. These 

 terminal regions are practically devoid of blood vessels, and 

 remain throughout the whole period of gestation as thin necrotic 

 areas which may closely adhere to, or become invaginated into 

 similar regions of adjacent vesicles. These, in virtue of their 

 necrotic structure, prevent actual fusions of chorionic tissues. 



Chorionic fusions may be occasionally found in swine, however. 

 Blastocysts may come to lie either side by side or end to end, and 

 then fuse. In each case obliteration of the intervening wall 

 results in the formation of a single chorionic cavity. Such 

 fusions are not often followed by vascular anastomoses, for the 

 vessels of the individual circulations do not often come into 

 close proximity. They may frequently be seen completely 

 reversing their direction upon approaching each other. The 

 total free-martin conditions are consequently of comparatively 

 rare occurrence in the pig. 



Investigation into this situation was prompted by the discovery 

 of a pair of heterosexual pig twins among some twinning material 

 kindly supplied by Dr. Newman, University of Chicago. 



I would like to express my thanks to Dr. Newman for kindly 

 placing his material at my disposal, and also for helpful advice 

 throughout this work. To Dr. R. F. Shaner, University of 

 Alberta, I am also indebted for valuable criticism, and to Swift 

 and Co. for cooperation during the collecting of material used 

 for this work. 



An examination this summer of approximately 400 uteri in 

 the stage of mid-pregnancy yielded 7 cases of twinning. These, 

 with 6 collected the previous summer, and 4 from the collection 

 of Dr. Newman, make a total of 17, of which 8 are judged to be 

 dizygotic in origin, 3 monozygotic, 4 undetermined, a pair of 

 conjoined twins and a double-headed monster. 



Seven instances of inter-embryonic vascular connections were 

 observed. In most of these, the anastomoses were of side 

 branches and therefore comparatively weak. In two cases the 

 twins showed a marked difference in size, accompanied by 

 injuries of the smaller individual. There was here, I take it, 



