Arthur Walton

Abstract The relative importance of heredity and environment in determining the size, development, and ultimate proportions of animals has attracted some scientific study and discussion, but little planned experimentation has been attempted with mammals. In the mammal the early and formative stages of development take place in the uterus, where the environment is remarkably constant, and it is not easy to subject the foetus to varying conditions and at the same time not overstep the normal physiological limits. Previous work on the rabbit (Hammond 1934 a; Wishart and Hammond 1933), however, has shown that the size of young at birth is approximately inversely proportional to the number in the litter, and it appears probable that some internal secretion or metabolic product of the mother forms a limiting factor in the growth of the embryo. In a monotocous species this effect does not appear, although exceptional twinning or polytocous births reveal its existence, the young at birth being regularly smaller. That the absolute amount of this hypothetical growth-promoting substance may be related in some way with the size of the mother is suggested by the figures of Gregory (1930), which show that the average number of ova of rabbits increases with the average size of the strain. In a monotocous species therefore the size of the mother might determine the size of the offspring at birth. This is also suggested by the results of reciprocal crosses between donkey and horse. The mule (donkey ♂ x horse ♀) is larger than the hinny (horse ♂ x donkey ♀) (Plumb 1916). Since, however, in a species cross the objection may be raised that abnormal genetic segregation might affect the result, we selected for our experiment two breeds of the same species, but of markedly different size, namely, the Shire horse and the Shetland pony, and reciprocal crosses were made to see how far the size of the mother would affect the size of the offspring. No genetical analysis of size inheritance in horses has yet been attempted, and therefore it may be said that the genetic potentialities of our material were unknown. In the absence of evidence to the contrary, however, we con­sidered it justifiable to assume, on the basis of much genetical experimenta­tion on size inheritance, that a large number of factors would be involved,that segregation and recombination would be at random, and that the off­ spring of the two breeds would be genetically intermediate, irrespective of which way the cross was made. Any difference between the offspring of the reciprocal crosses would therefore be due not to chromosomal differences but to differences in the environment brought about by difference in the size of the mother. In other words we would have a controlled experiment in which “Mother-size” was the only or predominating variable. The horse is particularly well adapted for studying the effects of maternal influence on the offspring, since the duration of pregnancy is relatively long (11 months) and the foal is born in an advanced stage of development, particularly with regard to the skeleton (see Meek 1901). Consequently, the effects of maternal influence acting over a long and formative period should be well marked.

PLASMA PHOSPHATASE OF CHICKS, 3 199 agreed within 12 %. For three preparations of vitamin D3, one in crude wheat flour and two in com oil, the potencies obtained by the phosphatase method differed from the value given by the boneash method by amounts varying from 25 to 40 %.

1. Certain characteristics of bull semen have been examined at four artificial insemination centres in England. One hundred and twenty-one samples of semen, comprising 168 ejaculates from seventysix bulls of seven breeds, were examined. The semen from 119 tested samples was subsequently used to inseminate 4604 cows. The following semen characteristics were studied: volume of ejaculate; concentration of spermatozoa; incidence of dead spermatozoa; incidence of morphologically abnormal spermatozoa; initial fructose concentration; resistance of spermatozoa to temporature shock; methylene-blue reduction; O 2 uptake; fructose utilization; visual evaluation of motility; impedance change frequency; and fertilizing capacity (conception rate). A clinical examination was made of the reproductive organs of each bull. 2. Variations in the concentration of living spermatozoa and in the incidence of dead spermatozoa were found to account for most of the variations in the metabolic activity of semon: they also largely explained differences in physical activity as assessed by visual estimation of motility or by impedance change frequency. 3. Although the rates of methylene-blue reduction, O 2 uptake and fructolysis showed a close correlation with the numbers of living spermatozoa present, O 2 uptake and fructolysis per living cell decreased with increasing cell concentration. 4. Variations in metabolic activity showed no correlation with variations in the morphological characteristics of the spermatozoa, except in the case of spermatozoa with bent tails. These forms occurred in some samples as a result of staining treatment, but their incidence was positively correlated with the O 2 uptake and fructolysis per living cell. 5. There was evidence that highly active spermatozoa are more resistant to temperature shock than less active spermatozoa.

ABSTRACT Ovulation has been directly observed in the anaesthetised rabbit. The follicle ruptures, on the average, about 10 hr. after coitus but there is some variation and all follicles do not rupture simultaneously. Ovulation is not the invariable consequence of the normal growth of the follicles and, although confined to those which are mature, is independent of the actual size attained. Ovulation is accompanied by increased vascularity of the follicle. The principal cause of rupture is the rapid distension of the follicle by marked secretory activity. The ovum is carried out in the flow of this secretion. Ovulation in the rabbit is initiated by some component of the orgasm which accompanies coitus rather than by the mechanical stimulus of the penis or the presence of semen in the vagina. Corpora lutea are not formed if the follicle is ruptured artificially and this effect is not due to operative trauma but to the absence of some effect of coitus necessary for the formation of the luteal tissue. Artificial rupture or ablation of ripe follicles is followed by an immediate compensatory growth of new follicles. Blood follicles formed from those artificially ruptured do not inhibit this compensatory growth.