PHYSIOLOGY OF REPRODUCTION IN DOMESTIC FOWL. 69 



The posterior of the two eggs (that is, the one toward the air 

 cell in the shell membrane) has a less deep but more viscous 

 envelope of thick albumen than the anterior one. In consequence 

 the anterior portion of the shell is larger. The relative position 

 of the chalazal axes of the two yolks cannot always be determined, 

 since, as noted above, the chalazae of the second yolk are often 

 not distinguishable. The chalazae of the first yolk are always, so 

 far as observed, in the same position as in a normal single-yolked 

 egg. A few cases have been observed where the chalazal axis 

 of the second egg was also in normal position. In one case, 

 however, where the chalazae of the second yolk were visible, the 

 chalazal axis was turned about 30 from normal. 



This group of eggs is the end of the series of double-yolked 

 eggs, since when an egg overtakes its predecessor after that egg 

 has passed entirely into the isthmus, the result is no longer two 

 eggs within one membrane and shell. The relation of this group 

 to twin eggs connected by a membranous tube is obvious. 



In all doubls-yolked eggs where one or more of the egg en- 

 velopes is not common to the two yolks, it seems that instead of 

 the simultaneous entrance of two yolks into the oviduct, the first 

 precedes the second by a short time. This is probably most 

 often due to the ovulation of a second yolk at a short interval 

 after the first. It may sometimes be due to the fact that either 

 the first or second yolk to enter the duct was ovulated into the 

 body cavity and was taken up by the duct shortly before or 

 immediately after the next ovulation. Or perhaps the first egg 

 may sometimes come to a place in the oviduct which is subnor- 

 mally sensitive to peristaltic stimuli, and remain there until joined 

 by the second yolk. There is also a fourth possibility, namely 

 that antipeiistaltic movements may carry the first egg back up 

 the duct until it meets the second. 



Parker 1 called attention to the fact that the occurrence of 

 antiperistalsis of the oviduct is proven in the cases where fully 

 formed eggs are found in the body cavity. He further shows 

 that only antiperistalsis can account for the enclosure of an egg 

 within an egg. If an egg which has received its shell membrane, 

 or its shell membrane and shell, is carried back up the duct and 



1 Loc. '/. 



