On the Nature of Hereditary Size Limitation

Abstract

1. New data are recorded for the post-natal growth in body weight of Flemish Giant and diminutive Polish rabbits, and of their F₁ hybrids. 2. The specific or percentage growth rate is identical for Flemish, hybrid and Polish males from the time of birth to the onset of puberty. 3. The process of development in the Flemish Giants differs from that of the Polish in that the latter are but half as large at the time of birth. The fact of their shorter gestation period (thirty days in contrast to thirty-one) does not entirely account for the difference observed. Hence the existence of some growth depressant acting upon the young of the small race (or growth accelerator acting upon the young of the large race) in utero may be surmised. 4. Hybrid animals from Giant does by Polish bucks and from the reciprocal cross are not so much affected, being at birth almost as large as the pure bred Giants. Accordingly the growth curves of hybrids and Giants closely approximate each other until the fourth month of post-natal existence. 5. During the fourth month Polish and hybrid animals undergo a more rapid retardation of growth than do the Flemish Giants. Their growth rate values, which have been indistinguishable hitherto, diverge at this time. The lesser size of the Polish adult can be ascribed to two factors, one effective during the placental period and the other at a more advanced age. In the last analysis both may have a common cause. 6. The growth curves of these rabbits have been described by the equation for autocatalysis sponsored by Crozier. These examples show the most extensive approximation to the theoretical of any mammalian data to which this equation has been hitherto applied. 7. In the equation two velocity constants are employed, each having its own magnitude in the large and small races. Although the hybrid offspring are intermediate in adult size their growth curves do not lie midwav between those of their parents. Prior to its inflection point the hybrid curve is similar in magnitude to that of the Flemish Giants, thereafter the early retardation of growth resembles that of the Polish. Moreover, the hybrid values of K₁ and K₂ are not both intermediate between those of the parents--K₁ being practically identical to that characteristic of the Polish parent. These facts invite extended investigation since they portend that in the inheritance of growth regulating processes there occurs a Mendelian segregation of unit characters. 8. Two small deviations of the observed from the theoretical values are consistently present and have been subject to analysis. The first growth aberration accompanies the circulatory, thermal and nutritional disturbances inevitably associated with birth. The second or juvenile deviation is concurrent with an endocrine reorganisation, in which the thymus, testes and suprarenal cortex are concerned. 9. The natal and juvenile growth retardations tend to divide the animal growth curve into three periods (one pre-natal and two post-natal) which have been dignified by the name of "growth cycles." Such a "cycle" does not represent an increase of the percentage growth rate but merely an increase of velocity due to the enlargement of a greater mass at a lesser rate. If one admit the validity of these observations the existence of disturbing factors is an adequate explanation of the three apparent cycles. Therefore it is maintained that to postulate a succession of autocatalysed master reactions is superfluous as well as inadequate. 10. Brody's conception of "five or more abruptly delimited periods of uniform increase" is not sustained by this material. 11. The adherence of these data to an integral curve descriptive of autocatalysis does not assure the validity of an analogy but the utility of that hypothesis has been enlarged.