CELLULAR l)E(TENEl{AriON AND EORMATION OE PIGMENT. 183 



same physiological condition with regard to age, nutrition, 

 reproduction, etc. 



The production of new formations, to use a medical term, 

 under the influence of adverse stimuli does not appear to be 

 a marked feature associated with pigment-degeneration in 

 hydroids; but in the case of Sertularia operculata there 

 seems to be a distinct hypertrophy of the interstitial tissue 

 resulting from an increase in the number of the rounded cells. 

 The nuclei of these cells are peculiar in not readily staining with 

 dyes and there may be no obvious nucleolus. The ectoderm 

 is rendered remarkably thick and the myo-epithelial cells are 

 mostly separated from the mesoglea, and appear as a somewhat 

 flat epithelium of large cells with conspicuous nuclei near the 

 surface. The ectoderm sheet around the hydranth is also hyper- 

 trophied (text-fig. 6, a). The constriction of the coenosarc is 

 probably connected with the multiplication of the interstitial 

 cells, and the pigmentation of these cells constitutes the essen- 

 tial feature of the phenomenon in this hydroid. We have seen 

 above that hypertrophy of the individual cells aifected fre- 

 quently occurs in that they may attain a size greater than the 

 normal cell appropriate to the kind of tissue of which they form 

 a part (text-fig. 4, r ; text-fig. 7, b, interstitial ectoderm cells). 



The action of intense light in inducing, or at any rate 

 accelerating, the diseases, pellagra, Kaposi's disease (Xero- 

 derma pigmentosum), and chloasma, and in the production of 

 freckles and of the general pigmentation of the skin is well 

 recognised. It is of interest to obtain evidence of an analo- 

 gous reaction in simple organisms. Curiously enough, in the 

 case of chloasma, different types of the pigmentation of the 

 skin in patches are recognised, corresponding to the influences 

 which undoubtedly must act energetically on hydroids 

 occurring in exposed rock-pools between the tide-marks of 

 the tropics ; these are ^ : 



(1) Chloasma sola re (Melasma sol are) due to exposure 

 to the sun or any powerful light such as reflection from 



1 Castellani and Chalmers, ' Manual of Tropical Medicine,' 2nd edit., 

 London, 1913, p. 1612. 



