92 GO. A. MAC MUNN. 
these latter are owing to the brevity of the description given 
of them. 
While I do not wish to minimise the importance of Merej- 
kowsky’s results, I feel bound to say that there has never been 
any valid reason given for considering any of the lipochromes 
included under the name tetronerythrin, by him, as respiratory 
pigments. And for this reason, that they fail to respond to 
the test used in determining whether any pigment is respiratory 
or not, namely, change of colour and spectrum under the 
influence of reducing agents. They are sensitive to light, and 
yield, in many cases, cholesterin-like substances, as Krukenberg 
has shown: an observation which gains additional significance 
when we compare an animal lipochrome with one of the most 
typical of that series among plants, namely, carrotin, which 
is accompanied by a cholesterin. Although, then, we now 
know, beyond all doubt, that Merejkowsky’s tetronerythrin 
includes under it rhodophan-, xanthophan-, and chlorophan- 
like lipochromes, and that some of the nearly related pigments 
which become changed into lipochromes, under the above- 
mentioned influences, are only lipochromogens, yet these 
results were of great value in showing what a wide distribu- 
tion such pigments have in the vegetable and animal kingdoms. 
Those colouring matters which, until recently, had been 
called luteins, among plants and animals, as well as by other 
names in the former, together with tetronerythrin, have now 
been included by Krukenberg! under the lipochromes. This 
useful generalisation was based on Kiihne’s researches? on 
the chromophanes of the retina. As is well known, Kihne sepa- 
rated out three pigments from the retina—rhodophan, chloro- 
phan, and xanthophan, and taught how to distinguish them 
from each other, and how to recognise such pigments when 
met with elsewhere. For although many earlier observers 
knew the reactions which these pigments undergo with nitric 
and sulphuric acids, and with iodine in iodide of potassium 
1 Loe. cit. 
2 Untersuchungen a. d. physiol. Instit. d. Univ. Heidelberg,’ Band i, 
Heft 4, 1878, and Band iv, 1882, 8. 169—248, 
