486 EMBEYOLOGY OF THE LOWER VEETEBEATES ch. 



body which are in proximity to it. Its living substance is every- 

 where bathed by — and no doubt intimately adapted to life in — an 

 internal medium, watery fluid laden with the products of metabolism 

 of the living substance as a whole. The -differences in function of 

 the various organs and tissues necessarily involve differences in their 

 metabolic activity and therefore differences in the chemical nature 

 of the contributions which they make to the complexity of the 

 internal medium as well as differences in the substances which they 

 withdraw from the internal medium for their own needs. 



Physiologists recognize that changes in the constitution of the 

 internal medium play an important part in exciting and controlling 

 vital actiVity. 1 In the ordinary life of the animal important examples ' 

 of such influence are. afforded by changes in the activity of the 

 normal function of an organ, as for instance when the pancreas 

 secretes actively in response to the presence in the internal medium 

 of a special substance secreted by the intestinal wall when stimulated 

 by food material. Other examples are afforded by changes in growth- 

 activity — as of the skin in response to a change in the amount of 

 substance secreted by the thyroid, or of the mammary gland in 

 response to the presence of substances produced by the metabolic 

 activity of the foetus. 



There is no reason to doubt that the living cells and tissues and 

 organs of the embryo are similarly adapted to and influenced by the 

 constitution of the internal medium, and if this be so the influence 

 in question must play an important part in development. A possible 

 example is afforded by the experimental result that the grafting of 

 the developing optic cup of an Amphibian embryo into near proximity 

 to ectodermal tissue (such as the pigment-layer of the retina, the wall 

 of the brain, the olfactory epithelium, the external ectoderm of the 

 head or trunk) is apt to induce that ectoderm to develop into a lens 

 (Bell, 1907). Such influence upon one portion of embryonic sub- 

 stance by another portion in its neighbourhood may well be exercised 

 through chemical or other changes produced by the specific meta- 

 bolism of the latter in the internal medium in its neighbourhood. 



A corollary to the consideration outlined above, which has an 

 important bearing upon much work in experimental embryology, 

 is that it is unwise to place reliance upon the mode of develop- 

 ment of an organ-rudiment being normal, unless its environment 

 is normal. 



(2) Cellulabization of the Zygote: Cellular Continuity 

 and Discontinuity. — Protoplasm being a soft semi-fluid substance a 

 particle of it, as it increases in volume during the process of growth 

 which is associated with normal metabolism, would soon reach a 

 mechanically unstable condition, in which retention of its character- 

 istic form, or even cohesion, would be impossible. In nature a 

 corrective to this is provided by the protoplasm undergoing fission. 

 In the Protozoa the products of this fission normally break apart and 



1 For their bearing upon evolutionary change see Parker (1909). 



