OSE FIGUREI n a cat in which tm-0 kittens werc born and then, 13 clays later, two more delivered, it was obscrved (hlarkee and Hinsey, '35) that the right uterine horn, which delivered first, had undergone involution, while the lcfl one was still bcing maiiitained in a pregnant state. This iiidicated R searcli f o r what may have been respoiisihlc €01-this difference, which occurred in spite of the fact that both horns werc subjected presumably to similar humoral influences. Our first approach to this was a n attempt to producc implantations in only one o f tlic two uterine horns and then to comparc thc occupied and tlie unoccupied hoi-ns in the same animal. This was doiic by interrupting one uteriiie tube iii a series of eight cats and, much t o our surprise, WP foiiiid hoth horns occupied iii seven of the eight aninials (hlarkee and Hinsey, '33)' indicating that in the cat internal migration of the ova can take place, as Corner ('31) and Warwick ('26) h a r e clcscribecl in the sow. However, the one cat in which the migration did not lake place will he described in detail, together with some other observations on thc cat which indicatc a local factor in uterinc growth.