ABSTRACT
In a previous paper (Berry, 1968) it has been demonstrated, by comparison of in vivo and in vitro growth of the rat foetus, that it is possible to dissociate growth and differentiation in the rat. In a series now exceeding 1000 embryos it is evident that for a given somite number the protein content of an animal developing in vitro might be less than half that of the normal control, suggesting that a considerable reduction in growth rate might be found without necessarily inducing death or malformation. The rate of somite formation is not reduced in vitro in the rat; it has been shown by Herrman & Schultz (1958) that the rate of somite formation is not interfered with by different explant conditions in the chick embryo. These findings suggest that the increase in somite number is more rigidly determined than the increase in growth rate.
