ABSTRACT
C3H/Bi and C57BL/How inbred strains of mice are known to differ from one another in number of lumbar vertebrae. Reciprocal F1 hybrids between the two strains tend to resemble the maternal strain in this respect. Since the difference between reciprocal hybrids is shown in females as well as in males, it cannot be attributed to sex linkage.
Analysis by means of egg transfer has shown that this reciprocal difference in the number of lumbar vertebrae is due to an influence exerted by the mother through the uterine environment, rather than through the cytoplasm of the egg.
The number of lumbar vertebrae is not affected by the operation of egg transfer in itself, but only by the specific influence of the host uterine environment.
In our material vertebral type is not influenced by the genetic constitution of the other embryos in the uterus, nor by their number. There is no marked dependence of number of lumbar vertebrae upon birth-weight.
We conclude that in this instance the uterine environment does not act through promotion or inhibition of overall embryonic growth, but exerts a specific morphogenetic influence upon the unborn young.
