A study of pre-natal growth and development in the sheep

The results are presented of an investigation in which the growth and development of forty single male sheep foetuses was examined. The material, obtained from slaughterhouses, included seventeen specimens of known age, while further details of the remainder were lacking. For these reasons, and the fact that the specimens were preserved in 10% formalin prior to dissection, relative, rather than absolute, trends were discussed. This had been achieved by expressing all values as percentages of those obtained from four single male lambs dissected at birth. 2. In order to age the experimental material with the greatest possible accuracy, data were collected from the literature on foetal weights and crownrump lengths of specimens of known age. Mean values were computed and normographs established for the pre-natal period between 18 days and fullterm. By employing the method of least squares, it was not possible to fit single curves to the data; in the case of both weight and crown-rump length the curves had to be divided into five segments in order to establish the relationships with age. Apart from a rather marked change in the rate of growth and development between the 109th and 110th days of gestation, the apparent ‘breaks’ in the curves were not considered important. It would seem that lack of data at certain stages, together with the heterogeneity of the population studied, were greatly responsible for the differential rates of growth and development recorded. 3. The data were inadequate to examine fully the effect of fixation on foetal weight; however, it would appear that whereas specimens weighing less than 200 g. and over 1200 g. lost weight during the course of preservation, foetuses intermediate between the above values gained weight. Since the causes could not be satisfactorily explained, the need for further investigation was emphasized. 4. Of the foetal measurements recorded, head length and, particularly, head width were shown to be earlier maturing than chest circumference and chest depth, while crown-rump length made even greater proportional increases during pre-natal life. In general, the results supported accepted theory that the retardation of rate of development proceeds in an anterior-posterior direction prior to birth. 5. By dissecting the head from the body and weighing each unit separately, it was shown that the head decreases in relative size throughout foetal life. Head:body ratio, however, was found to be unsuitable as a criterion of pre-natal age, since foetuses of equal age may differ appreciably in this respect. It was shown, for example, that a lamb at birth, though above the average in terms of absolute weight, may possess the conformation of a 120-day-old foetus on the basis of head: body ratio. 6. Dissections of the major constituent parts of the foetal body indicated the skinned head to be the earliest maturing, followed in order by the organs (weighed en masse ), dressed carcass, total skin and, finally, the skinned feet and tail, which were latest maturing. The order in which these parts grow before birth appeared explicable primarily upon a basis of functional necessity. 7. The skull was shown to develop to a relatively lesser degree during foetal life than the mandibles, while skull measurements indicated the cranial portion to be earlier maturing than the facial. Linear measurements of the various vertebral regions at different stages of development did not yield entirely satisfactory results, but it was evident that the anterior cervical vertebrae are earlier maturing than those of the posterior extremity (caudal vertebrae). 8. Weights of the major units of both thoracic and pelvic limbs indicated the former to be slightly earlier maturing. In contrast to the direction of retardation of growth during post-natal life, in the foetus the gradient followed a proximo-distal direction; scapula and pelvis, for example, being earlier maturing than metacarpus and metatarsus. 9. Linear development of individual bones was shown to be earlier maturing than growth in weight. However, on the basis of length measurements, the course of development proceeded in a similar direction as the trends established in terms of weight, i.e. proximo-distally. Width of pelvis, on the contrary, was shown to be earlier maturing than width of scapula; this was explained in terms, particularly, of earlier developing cartilage in the former bone. 10. In terms of weight, m. longissimus dorsi appeared to be earlier maturing than m. rectus femoris and m. gastrocnemius lateralis; between the latter two muscles no marked difference in rate of growth could be established. The results indicated that these three muscles develop at much the same rate in respect of length and depth during pre-natal life, but m. longissimus dorsi showed relatively less development in width over the same period of time than the other muscles. 11. Of each of the above-mentioned muscles, the cross-diameter of fifty individual fibres was measured by means of an ocular micrometer. Again, a tendency was observed for fibres of m. longissimus dorsi to be earlier maturing than those of m. rectus femoris or m. gastrocnemius. 12. Development of muscle-fibre diameter during foetal life was examined further on the overall mean for the three muscles studied, i.e. the mean of 150 measurements per foetus. The results showed that whereas muscle-fibre diameter increased but slightly (approximately 19·5%) during the first two-thirds of pre-natal life, the increase thereafter was substantial; on the basis of a linear regression, the increase amounted to 113·2% from about 108 days to full-term. The data thus appears to support the accepted view that muscular growth occurs initially by an increase in the number of fibres, and in late foetal life primarily by hypertrophy. 13. The dispersion of muscle-fibre sizes, in absolute measure, was shown to increase with advancing foetal age, the range at birth being considerable. In accord with the results of other investigators, a small proportion of small fibres was observed between approximate ages of 61 and 68 days which, apparently, were not present at either earlier or later stages. 14. Measurement of fibres from muscles situated in different anatomical regions of the body indicated that the muscles of the head were earlier maturing than those of the trunk, and muscles of the thoracic limb earlier maturing than those of the pelvic limb. In agreement with the results obtained on bone weights and measurements, the retardation appeared to proceed in a proximo-distal direction, particularly in the case of the pelvic limb.