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THE AMEBIC AN NATURALIST [Vol. XL V 



As I have pointed out above, the most interesting ques- 

 tion regarding this egg pertains to the physiology of its 

 formation. Parker states that two hypotheses have been 

 advanced to explain how inclosed double eggs are formed. 

 According to one of these, which was first advocated by 

 Panum, 3 the inclosed egg remains in the distal part of 

 the oviduct until overtaken by a second one, when both 

 are then surrounded by a common envelope ; according to 

 the other a completely formed egg is carried by antiperi- 

 stalsis back up the oviduct, where it meets a second one, 

 and the two passing down become covered by a second 

 shell and are laid. It seems quite evident from the de- 

 scription of the egg just given that it is the product of 

 antiperistalsis, but the especial interest lies in the fact 

 that this process has taken place twice. 



The first antiperistalsis took place immediately after 

 the hard-shelled egg was formed, and of course caused its 

 migration to the upper or proximal end of the oviduct 

 where it met the second egg. This meeting must have 

 taken place very close to the infundibulum, for otherwise 

 the yolk of the second egg would have possessed much 

 larger chalazae. 



The second antiperistalsis occurred immediately after 

 the inner of the two shell-membranes had been laid down, 

 and must have succeeded in carrying the double egg up 

 the oviduct to a point where albumen is secreted, that is, 

 to a place slightly above the beginning point of the isth- 

 mus ; for it is only on this assumption that we are able to 

 explain how a thin layer of albumen came to exist be- 

 tween the two shell-membranes. The small amount of 

 lime deposited on the outer of the two shell-membranes 

 indicates that the egg did not remain long in the uterus, 

 but must have been laid shortly after having entered that 

 organ. 



In many respects this egg conforms to the facts already 

 seen in the inclosed types of double eggs; thus the in- 



