68 MAYXiE R. CURTIS. 



of the first. It is not always possible to determine the relation 

 of the two yolks by this means. The many cases where this is 

 possible, however, show that the chalazal axes of the two yolks 

 are related to each other in various ways. Immerman 1 has shown 

 by incubating double-yolked eggs that this is also true of the 

 embryological axes. A study of double-yolked eggs suggests 

 that this variation in position of the axes may in part result from 

 the difference in the time elapsing between the entrance of the 

 two yolks into the duct. 



The next group of double-yolked eggs includes the transitional 

 form between the eggs that have yolks with separate chalazae 

 and a single thick albumen envelope and those with yolks that 

 have separate albumen envelopes. This group is not always 

 easily distinguished from the last group and it is often impossible 

 to decide whether or not one or both of the yolks have some 

 separate layers of albumen. There are, however, cases where 

 there is certainly considerable thick albumen between the two 

 yolks although both are enclosed in a common mass of thick 

 albumen. This type of egg will be produced whenever the 

 second egg overtakes the first in the albumen secreting portion of 

 the duct. 



The last group of double-yolked eggs to be considered contains 

 those which have separate thick albumen envelopes. This group 

 is easily distinguished and is not uncommon. Seven of the 

 44 double-yolked eggs produced by the station flock during the 

 month between September 20 and October 20, 1913, belong to 

 this group. These eggs can usually be distinguished by the 

 shape of the shell which has more or less of a depressed ring 

 around the center. The double nature of the egg is visible 

 externally. The depressed ring is sometimes hardly visible, or 

 again it may be very pronounced. In cases where the depression 

 is slight there is usually only a smooth membrane inside the shell, 

 but in cases where it is more pronounced there is some limes at 

 the base of the depression a very narrow shelf of membrane which 

 extends all or part of the way around the shell. There can be 

 no doubt that in this group the two eggs join while passing into 

 theisthmus and that the depth of the depression depends upon 

 the time during this passage when the two eggs unite. 



1 Loc. cil. 



