66 MAVXIE R. CURTIS 



The two yolks of a double-yolked egg often possess some sep- 

 arate envelopes, indicating that they have not passed through 

 the entire length of the duct together. In fact a careful study 

 of a large number of double-yolked eggs indicates that the second 

 may join the first at any point between the infundibulum and the 

 isthmus ring. 



Two separate yolks with a common set of envelopes may be so 

 closely united by the chalaziferous layer that they appear to have 

 also a common vitelline membrane. Such pairs of yolks are 

 very much flattened together. There is a complete and graded 

 series from this condition to one where the two yolks barely 

 touch one another. In the latter case the two yolks have really 

 chalazal membranes which are practically separate although they 

 still have but one set of chalazae. The two yolks have evidently 

 passed down the duct in quick succession but the first has 

 preceded the second sufficiently to allow each to preserve it& 

 form. 



This group of double-yolked eggs passes insensibly into the 

 group in which the yolks have separate chalaziferous layers but 

 all the other envelopes in common. Fig. I shows diagrammatic- 

 ally the typical relations of the two yolks in double-yolked eggs 

 where the yolks have common albumen envelopes. 



Where the arrangements of the two yolks and the chalazae 

 are those shown in Fig. I, c to f, the second yolk must have 

 overtaken the first after the chalazal layer was formed and before 

 the thick albumen was secreted; that is, near the beginning of 

 the albumen secreting portion of the duct. In the cases figured 

 the stimulations given by the two yolks upon the glands which 

 cause the formation of the chalazae must have overlapped. In 

 cases like Fig. I, / the chalazae sometimes appear thin in the 

 middle. This may be due to the faintness of stimulation on the 

 glands or to stretching of the chalazae as the yolks are bent on 

 their axes. A few double-yolked eggs with one set of albumen 

 envelopes have been observed where the two yolks had separate 

 chalazae and several where one yolk had two and the other had 

 none that were visible. It should be stated that in all types of 

 double-yolked eggs the chalazae of the second yolk, (i. e., the one 

 nearest air cell of the egg) are often less well developed than those 



