Fertilized ova have been transferred from Border Leicester to Welsh Mountain ewes, and vice versa. The oestrous cycles of the ewes were synchronized by progesterone treatment, and donor ewes were super-ovulated with PMS. 68–90 hr. after the onset of oestrus in the donors, thirty-seven ova, representing 58% of the ovulations, were recovered in vivo from thirteen ewes. Nineteen ova were transferred to eighteen recipient ewes and a total of eight pure-bred lambs were born to ewes of the opposite breed.Transfers resulted in pregnancy when oestrus in the recipients commenced from 16 hr. before to 20 hr. after that in the donors; two of the ova which developed to term were 4-cell, five were 8-cell and one was a 16-cell ovum at the time of transfer. Four out of seven and four out of eleven transfers, using blood serum and Ringer's solution respectively, resulted in the birth of lambs. Prior to transfer to the recipients seven of the ova which developed to term were exposed for 30–45 min. at room temperature, whilst one developed after exposure for 70 min.

With Plates 1-3 and Sixteen Text-figures)Factors which affect the growth of the young animal are of great economic importance in animal production. In the past, much experimental work has been concerned with the effect on the growth of the young animal of different environmental factors. Many of these factors do not, however, affect the development of the young animal directly, but rather act on its maternal environment. Thus, much is known of the factors which affect the milk secretion of the female and this in turn is known to have a considerable effect on the growth of the young; similarly, the level of nutrition of the female during pregnancy has been found to influence foetal growth.The maternal environment may affect the growth of the offspring at two stages. Before birth, the foetus is entirely dependent upon its mother for its nutrition, so that foetal growth is subject to her control. After birth, the young animal is at first entirely dependent and, for a considerable period, is largely dependent on maternal care, or its equivalent, a most important feature of which is the milk supply available. The maternal influence may be defined as the sum of the effects of those maternal factors which influence the growth of the young after fertilization of the egg; it is made up of pre-natal and postnatal elements. Walton & Hammond (1938) and Joubert & Hammond (1954) have studied the effect of the maternal environment in horses and cattle respectively, by crossing breeds which differed greatly in size. Venge (1950) has studied the maternal influence on size in rabbits both by crossing breeds of markedly different size and also by transferring fertilized eggs between these breeds. The two methods used by Venge were also employed in the experiments to be described in this paper. These experiments were based on suppositions similar to those on which the above-mentioned earlier experiments with other species were based, namely, that breeds of ewes which differ markedly in size would provide different maternal environments, and that the cross-bred lambs would have similar genetic constitutions irrespective of which way the crosses were made. The control of body size and growth by sex-linked genes is thus assumed to be very small in relation to the main factors studied in these experiments.The two breeds of sheep which were selected for the present experiments were the Border Leicester and the Welsh Mountain; the live weight of mature Border Leicester ewes averages 140-180 lb., whilst that of Welsh Mountain ewes averages only 80-100 lb. In the first experiment, these breeds were crossed reciprocally so that (cross-bred) lambs having similar genetic constitutions were reared in the very different maternal environments provided by the two breeds of ewe; the effects on the size of the lambs of the two environments could then be compared. By using young and mature ewes, it was also possible to estimate the influence of the age of the mother on the size of the lambs. Thus the effects of (a) breed of ram, (6) breed of ewe, (c)...

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To investigate the relationship between oestrus and time of ovulation from the start of heat and the effect of the ram on this relationship, 200 ewes, half of which had been run with rams continuously during oestrus, were slaughtered at intervals after the end of heat. The method of probit analysis was used to fit multiple regression equations for percentage of ewes ovulated (as a probit) in terms of slaughter time and heat length. From these equations, estimates of mean ovulation times for various heat lengths were calculated. In the control group, which was teased at 4-hr intervals to determine lengths of heat, ovulation occurred, on the average, shortly after the end of oestrus. Although duration of oestrus was shorter when the sexes were continuously associated, ovulation occurred later. In addition, there was a positive correlation between heat length and time of ovulation in this group, but not in the control group.