4. DR. HICKS, ON FRESH-WATER ALG#, ETC. 
in action in these ova. No growth by opposition of layers, 
either from the inside or outside, either by hardening of an 
exudation from, or by conversion of the substance of the 
yelk into that of the yelk-sac, can explain the increase in 
number and size of the buttons on the outer surface, and of 
the dots in the substance of the yelk-sac. There are other diffi- 
culties against the acceptance of the usually received view, 
which will at once occur to any one who considers the 
arrangement of the dots and lamine; but I have said enough, 
I think, to justify the inference that the dotted yelk-sac of 
osseous fishes grows in some way by interstitial molecular 
deposit. This view I am disposed to extend to other analo- 
gous tissues, perhaps to the so-called intercellular matrix of 
cartilage. 
A word or two on the methods which this sort of inquiry 
demands. A medium is wanted which separates the different 
objects, and is as far as possible without influence on the 
optical or other properties of the tissue. But such a medium 
is, perhaps, unattainable, as each part of the egg differs from 
the other in its reactions to media. On the whole, the best 
fluid in which to conduct an examination of this kind isa 
weak solution of glycerine, such as is found by experiment 
not to alter the aspect of the red blood-corpuscles of the 
animal. The plan of staining tissues by carmine suggested 
by Dr. Beale is not to be recommended; for the ammonia 
rapidly dissolves the germinal vesicle and its contents, and 
the acetic acid glycerine obscures the finer markings upon 
the yelk-sac, as well as deforms the yelk. Moreover, I may 
mention that the granular formative yelk takes the dye with 
greater difficulty than the yelk-sac does, except in very young 
ova, and the inner sac, a true germinal matter, does not take 
any stain; so that I cannot accept the staining of certain 
parts of a structure as satisfactory evidence of the distribu- 
tion of germinal matter in the tissue. 
On FRESH-WATER ALG, &e. 
By Dr. J. Braxton Hicks, M.D., F.RS., F.LS., &c. 
My principal motive in remarking on Mr. Archer’s paper 
on Palmoglea macrococca was not so much to question the 
independent existence of the forms of which he wrote, as to 
urge him to direct his unfettered attention to the study of 
