114 PROaEESS OF MICROSCOPICAL SCIENCE. ['YourS, A^Triff 
the glass whicli forms the upper part of the gas-chamber. In such 
cases the air-bubbles preserve their size and form for a long while ; at 
the same time the protoplasm is extremely mobile, and its expansions 
are frequently disappearing, like a dissolving-view. The form of the 
air-bubbles is irregular, being never perfectly spherical. When a 
current — that formed by making contact — is passed through the drops, 
the immediate, or nearly immediate, result is, that the bubbles of air 
become perfectly spherical, or — in places where the "test" prevents 
them— spheroidal, and the protoplasm withdraws a little from the 
inner surface of the " test." After a while — from one to ten or 
twenty seconds — the form of the vesicles becomes irregular, the proto- 
plasm recommences its movements, and the protuberances reappear. 
After some minutes further, the bubbles of air become smaller and 
smaller, and the Arcella descends. Soon, however, in about half a 
minute, the bubbles become enlarged again, and the Arcella reascends. 
M. Engelmann says the experiment may be repeated several times 
with the same results. The author concludes his remarks by saying : 
— " From the fact of the bubble of air becomiug spherical during the 
contraction due to the electrical action, it follows that the protoplasm 
during its contraction obeys the mechanical laws of liquids. The air- 
bubbles in becoming spherical do not apparently alter their volume." 
Tlie JDevelopment of Acaridce. — At a recent meeting of the Royal 
Academy of Belgium, M. Van Beneden read a letter which he had 
received from M. Bessels, of Stuttgard, relative to the above subject. 
The substance of the letter was briefly as follows : — " The embryogeny 
of the Arthropoda had been very little studied up to the year 1863. 
But since the appearance of Weissman's work on the development of 
the Diptera, the subject has received more attention, and has been 
especially brought out by the memoirs by Mecznikow and Dohrn ; and 
lately I have received from my friend Dr. Alexander Brandt, of St. 
Petersburg, a memoir published by the Academy on the subject of the 
development of the LihelluUdce and Hemiptera, in which the author deals 
especially with the embryonic membranes. For some time I have devoted 
attention to the development of Atax, Phytopus, Sarcoptes, Tetranychus, 
and other kindred genera." The writer then observes that his observa- 
tions entirely agree with those of Claparede, published in a late number 
of Siebold and Kolliker's Zeitsch-ift, and he states that his results do not 
accord with those of M. Van Beneden. " I have," he says, " been equally 
unsuccessful, with Claparede, in not finding the germinal vesicle in 
ova recently deposited, as you have announced in your researches on 
the development of Atax ypsilophorus (p. 18). Doubtless it escaped 
my observation owing to the deep colour of the vitellus. Having 
never observed the ovarian eggs, it seems to me impossible to decide 
whether the membrane which surrounds the vitellus is a vitelline 
membrane or a chorion. I have been fortunate enough to observe the 
mode of formation of the blastoderm, which both you and Claparede 
missed. It would be hard to say how soon the blastoderm appears 
before the ovum leaves the oviduct, since the deposition of the ova 
has not been observed. In ova taken from the mantle of TJnio and 
Anodon I have seen the blastoderm appear in from one to two days. 
If one of these ova be opened cautiously in a solution of one per cent. 
