184 EDWARD MCCRADY, JR. 



but with only a small aperture to the outside. When the soft 

 nipple is sucked through this narrow opening it swells up on 

 the inside and 'buttons' the young to the mother. For several 

 days this is a weak attachment, but the more the nipple is 

 sucked upon, the more it enlarges at its tip, and the more 

 secure the attachment becomes. After about the fourth day in 

 the pouch the young cannot be removed without considerable 

 effort and danger of tearing the corners of the mouth. This 

 is not due to any organic union between maternal and foetal 

 tissues, as has been popularly supposed, but simply to the 

 fact that the oral aperture is not large enough to permit the 



Fig. 57 Photograph of section from an 8-day post partum opossum with the 

 mammary nipple still in its mouth. No fusion of maternal and embryonic tissue 

 occurs. 



passage of the bulbous end of the nipple. Figure 57 shows a 

 sagittal section through the head of a young mammary foetus 

 with the nipple in the mouth. 



There is also a traditional belief that the young swallows 

 the nipple all the way down to the stomach. This is based 

 upon the observation that after about 6 or 7 weeks of nursing, 

 the nipple stretches to some 14 inches in length. But only 

 the tip of this is ever in the young's mouth. The epiglottis 

 is so constructed (v.i.) that the nipple cannot pass it. The 

 long nipple allows the advanced young to nurse from outside 

 the pouch when they have become too large for the pouch to 

 contain them all at once. 



