Biological and statistical studies on the New Zealand Romney lamb with reference to relative growth gradients

Abstract

I) A series of eighty -three dated foetuses of the few Zealand Romney breed of sheep have been used to provide material for a study of the relationship between increase in body size and development and arrangement of wool follicles and the effects of these on the characterisation of the fleece in different body regions.2) The results of preliminary studies made on fibre - type- arrays, and on follicle arrangement are incorporated in two papers, which are presented in support of this thesis: viz: Galpin I and Galpin II in appendix.3) . It has been shewn that different regions of the body show different growth,.rate increases relative to crown -rump measurements. The foetal head showed a decrease in relative growth-rate from the 74th day to the 122nd day of intra-uterine life; the forelimb region showed a slight increase in relative growth-rate for length and width after the 86th day and continued to grow at approximately the same rate until the 122nd day; the hind - limb showed a marked increase in relative growth -rate from the 86th day and continued to grow at approximately the same rate until the 122nd day.4) The formula y = bxα, which has been proposed by Huxley to replace y = bxᵏ, has been used to determine differences in rates of growth of different body regions.5) It was found that the equilibrium constant 'α-with-time' exaggerated the magnitude of small but significant variations in growth rates, which might be somewhat obscured by results calculated from the formula: α=(log.y₁-log.y) / (log.x₁-log.x)6) From the values of the equilibrium constant α for the head fore and hindlimb regions, it was found that there was an antero -posterior gradient for relative growth -rates - the anterior regions growing pore slowly than the posterior ones. further it was determined that this antero- posterior gradient was slight previous to the 86 days stage of development and marked after the stage.7) The antero- posterior growth -rate gradient was found to be comparable with the postero- anterior (Britch -poll) fibre- type -array gradient; coarse- fibred arrays occurred posterior to fine-fibred arrays.8) It has been shown from the changes in value of 'α - with -time' for the different regions, from period to period that the entero- posterior gradient for relative' growthrate was not obvious in the early stages of development and did not become important until about the 80 days stage There was a gradual change from stages with no gradient to stages with a marked gradient.9) In each region the varying growth rates have been correlated with local fibre population density, and it has been shown that acting together these may both have an effect upon the general character of the wool grown on a region.10) It has been shown that positions on which follicle initiation begins early have higher percentage counts of pre-curly -tip fibres than those on which follicle initiation begins later. Correlation with relative growth rates showed that the former regions grew more rapidly until the completion of the trio stage than they did after it. Positions with low pre-curly -tip counts showed an initiation of follicle growth at a time when the growth rate of the region was slower than it would be during the later stages of follicle development.11) Coarse curly -tip fibres were found on regions which grew rapidly after the completion of the trio stage - notably the britch and superior ileum positions. The britch had a pre- curly -tip count the same as the withers, i.e. midway between that of the poll and superior ileum positions;' this may be explained by the fact that, though on both britch and withers follicles commence to grow at the same time, the forelimb regions did not show an increase in growth rate after the completion of the trio stage comparable with that of the hindlimb regions where th increase in growth rate was very marked.12) It has been shown that formation of glands and ducts takes place about three weeks after initiation of follicle growth, and that fibres pierce the skin between six and seven weeks after their follicles were first laid down.13) The evidence accumulated permits the enunciation of the hypothesis that there is a direct correlation between rates of growth and follicle densities in each region, and that the ultimate local characterisation of the fleece depends upon the relative degrees to which these operate. The faster /region grows after follicles are first laid down and the less the follicle density, the coarser is the wool grown upon it; similarly, the slower the growth and the greater the density, the finer is the wool produced.