678 JULIAN 8. HUXLEY 
Immersions of the fertilized egg in solutions of poisons so 
dilute as to allow development to proceed, while yet exerting 
an influence on the more susceptible parts of the organism, 
give the following results. (Essentially similar facts were 
discovered for other Polychaetes (Nereis and Arenicola).) 
Immersion continuously up to the late larval stage gives 
a form with both anterior and posterior regions smaller and 
less differentiated than the normal. The middle region is either 
almost as large, and of the same form as the normal, or else 
considerably distended. This latter condition implies possibly 
that the cells of this region have been able to develop practi- 
cally normally. The anterior and posterior regions are not so 
active as normally, and hence are not able to make use of so 
much of the yolk; there is thus more for the middle region, 
which is capable of utilizing it, and secretes an excess of fluid. 
If immersed for eleven hours only, and then replaced in sea- 
water, the apical region is small, but the growing region as well 
as the middle region is nearly normal. If, on the other hand, 
the development is allowed to proceed in sea-water for twelve 
or twenty-four hours, and the larvae are then placed in the 
solution, the apical region, having been completed before 
immersion in the toxic solution, is normal and the posterior 
end is much affected. 
in another paper, giving an account of similar experiments 
on Echinoderms, he makes an interesting suggestion to account 
for the great over-development of the skeleton often found in 
larvae which have grown in dilute solutions of toxic agents. 
The mesenchyme cells appear to be least susceptible, and thus 
when the other cells of the organism are inhibited, can obtain 
a greater quantity of food, which results in a multiplication 
not only of themselves but of the products of their activity, 
i.e. the skeleton (Child, 1916). 
A recent important attempt to apply similar principles has 
been made by Robertson and Ray (1920, where reference to 
earlier papers are given). 
Robertson found that mice to whose diet had been added 
tethelin from the anterior lobe of the pituitary, showed first 
