During the course of investigation into the relationship between the growth of supporting tissues and parenchyma, it was observed that there was a rapid formation of collagen in the rat's uterus during pregnancy and, a fact which was in some ways more interesting, that this collagen disappeared very rapidly from the uterus after parturition . As a preliminary to further work on this striking post-partum change, it was necessary to have more accurate information on the time-course of disappearance of collagen. This we have now obtained.It was also necessary to have some information about the route of loss of collagen. The principal interest in the disappearance of collagen from the uterus after parturition lies in the assumption that it represents an internal reabsorption of the material. It has been suggested several times in discussions of our earlier findings that this may not be so, and that collagen may be shed into the lumen of the uterus and lost through the vagina. We have investigated this point.A short account of this investigation has already been published (Harkness & Moralee, 1955). METHODSThe rats were albinos of the local stock about 3 months old and weighing 180-200 g at the start of pregnancy. All pregnancies were first pregnancies. The rats were placed in individual cages 3 days before the expected time of parturition. When they were required for examination of tissues they were killed by a blow on the head, and breaking the neck. The whole uterus was removed, the suspensory ligaments being cut off flush with the wall. The cervical part and the horns were cut apart, opened, blotted rapidly on filter-paper and weighed. The collagen content was then estimated by the method of Neuman & Logan (1950a).Samples of material going from the uterus to the vagina were obtained from other animals by means of cotton-wool pledglets (cylindrical dental swabs 1 cm long by 3 mm diameter) covered with one layer of surgical gauze. These were introduced into the vagina without anaesthesia through a glass tube and left in for 6-18 hr. They were removed with forceps, placed in 5 ml. of distilled water and autoclaved for 6 hr at 30 Lb./sq.in. (2.1 kg/cm2). The watery extract was

It therefore seemed of interest to study collagen formation in the uterus in other circumstances. We have now studied the oestrous cycle and found no significant variation in the collagen content of the uterus at different times. We have also studied the effect ofoestrone and progesterone on collagen formation in the uterus of spayed animals and have found it to be slight compared with the effect on total growth of the uterus. Under these circumstances, the behaviour of the uterus is unlike its behaviour in pregnancy and resembles rather the behaviour of other tissues subjected to a stimulus to growth. A preliminary account of some of this work has already been published (Harkness, Harkness & Moralee, 1955). METHODSThe animals were young adult albino rats ofthe local strain. Whenrequired forexamination oftheir tissues they were killed by a blow on the head and breaking the neck. The uterus was removed and subdivided into horns and cervical parts, which were examined separately.Oestrous cycle. The stages of the oestrous cycle were identified by vaginal smears made once a day in the morning. Several cycles were followed in each rat before it was killed for examination. The majority of the cycles had a length of four days, though some had a length of five. Rats showing longer cycles were not used.

1. Loss of collagen from the uterus of the rat after spaying, and from a non‐pregnant horn after parturition has been investigated. 2. Loss of collagen from a non‐pregnant horn after parturition proceeds rapidly, at about the same rate as loss of collagen from the uterine cervix after parturition. 3. After spaying, loss of collagen from the uterus proceeds much more slowly than after parturition, and more slowly than loss of weight and non‐collagenous protein N. 4. An incidental finding was that the number of foetal sites in the horn of the opposite side after unilateral spaying was approximately the same as in both horns, i.e. double that normally found in a single horn.