PROGRESS OF MICROSCOPICAL SCIENCE. 
91 
(probably) Nostoc paludosum. This species is a very minute one, 
though tbe dimensions of the subglobose or elliptic fronds vary much. 
It is rather common in moor and certain bog pools. On account of 
its small size, therefore, readily capable of compression, and its 
pellucid character, the elegant arrangement of its tortuously-twisted 
rather large moniliform filaments, is often nicely seen, and this causes 
it to be a very pretty and favourable illustrative example of its type 
for examination in its entirety under the higher powers of the micro- 
scope. Its minute size calls to mind Nostoc minimum (Currey), but in 
it the cells are described as quadrate with a sinus at each side, lending 
a crenate outline to the filaments, and the heterocysts are large, 
•whilst here the cells are orbicular, or for a time slightly flattened at 
the junctions, and the heterocysts are but slightly wider, though 
longer than the ordinary cells. This plant is probably identical with 
Nostoc paludosum (Kiitz.), though as regards anything to be deduced 
from the lieterocysts Kutzing is silent. But the interesting point 
connected with it is a single example of it having presented indu- 
bitable “ spores,” and precisely similar nature to those in Sphaerozyga, 
&c., but with the peculiarity of these being always placed singly 
between two heterocysts. The pairs of lieterocysts with the inter- 
vening spore occurred at just about the same intervals as in ordinary 
examples occur the isolated heterocysts ; the spores large, broadly 
elliptic, about one-third longer than broad ; their diameter more than 
twice the diameter of the heterocysts, about thrice the diameter of the 
ordinary cells ; the “ bright points ” of the heterocysts not very 
conspicuous. 
Development of the Shull of Salmo salar. — Our President, Mr. W. 
K. Parker, F.E.S., has just given the Royal Society a most elaborate 
and wonderful account of the development of the skull of this fish. 
We cannot be expected to do more than give an outline of the author’s 
mass of facts so patiently and accurately worked out. His last subject, 
the frog, being fairly out of hand, Mr. Parker says that he set himself 
last summer to this “ newer and more easy task — more easy by far, for 
the translucency of the young salmon contrasts most favourably 
with the obscurity of the embryo frog.” He found that the two types 
at the time of hatching did not start fairly, but that the salmon had 
hastened to finish its fourth stage before emerging from the egg ; this, 
however, is partly in consequence of the difference of the envelope in 
which the embryos are contained, for in the salmon this is a leathery 
“ chorion,” and in the frog a mere gelatinous bleb. Moreover, it soon 
became apparent that these two “ Ichthyopsidans” are in nowise near 
akin to each other. In the very first stage, where there is an essential 
agreement, in one important particular they greatly disagree ; for the 
embryo of the salmon has two arches in front of its mouth, while the 
tadpole has but one; there is also an additional gill-arch in the osseous 
fish. In the earliest stage of the salmon worked out by him he found 
a much more distinct condition of the parts than in frogs at the same 
stage ; the differentiation of the latter is obscure as compared with the 
fish, and this not merely because of the quantity of pigmentum nigrum 
in the tissues of the former. Then, in addition to other causes of 
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