SCIENTIFIC SUMMAEY. 
219 
When the gas was ultimately expelled, the loss, as ascertained by direct 
weighing, was 0’01164 gi*m. The charged wire measured 618-923 millims., 
showing an increase in length of 9-779 millims. (0-385 inch). The increase 
in linear dimensions is from 100 to 101-605 ; and in cubic capacity, assuming 
the expansion to be equal in all directions, from 100 to 104-908. Supposing 
the two metals united without any change of volume, the alloy may there- 
fore be said to be composed of : — 
By volume. 
Palladium 100 or 95-32 
Hydrogenium 4-908 or 4-68 
104-908 100 
ZOOLOGY AND COMPAEATIVE ANATOMY. 
The Formation of the Blastoderm in Crustacea. — The fine memoir of MM. 
Van Beneden and Bessels on this subject, whicli was lately presented to the 
Academy of Belgium, and an abstract of which was given by the authors 
in the Monthly Microscopical Journal, has been reported on to the Academy 
by the veteran physiologist Schwann. The following is a part of M. 
Schwann’s report. With regard to the formation of the blastoderm in the 
Crustacea examined, the authors distinguish two types: in the first, the 
blastoderm is preceded by the total segmentation of the vitellus, which first 
divides into two portions, each portion again dividing into two, etc. The 
authors have occasionally observed a globe divide itself into four portions 
instead of two. In this case the nucleus first divides itself into four parts. 
The last globes which result from these successive divisions, and which, in 
my opinion, are true germ cells without membrane, are of a pyramidal 
shape the base of the pyramid is inclined towards the surface of the egg, 
towards the chorion, the apex towards the centre. They are full of nutrient 
globules. Before the segmentation is completed, the globules pass towards 
the apex of the pyramids, then towards the centre of the egg, while the 
base of the pyramids, which touches the chorion and contains the nucleus, 
becomes transparent. This base finally separates itself by stricture from 
the summit, and constitutes the blastoderma, while the apices form a nutrient 
non-cellular mass — the plasma — in the blastodermic vesicle. This plasma 
may be subsequently split up by a totally different operaijon to that of the 
segmentation of the vitellus. In the second type of the formation of the 
blastoderm very different phenomena present themselves. At a given 
point of the surface of the vitellus a few large cells present themselves. 
They multiply by division, commencing with the nucleolus ; then the nucleus 
divides ; and then the protoplasm of each cell. These cells being but few 
in number, form a small zone under the chorion ; this zone spreads, and 
finally encircles the vitellus, which is thus placed in the centre. M. 
Edouard van Beneden, by an ingenious theory, succeeds in reconciling these 
two types of the formation of the blastoderm, although they appear to 
differ so widely. The vitellus is composed, as we have already said, of the 
protoplasm of the cell-egg and of the plasma — that is to say, of the nuti-ient 
