*J?iliL. Oc^fS^ PROGRESS OF MICROSCOPICAL SCIENCE. 209 
A number of questions remain to be decided: — (1) Is tbe 
ovum of Asellus really covered at the moment of deposition by- 
two envelopes, as MM. Dohrn and Sars allege? (2) What is 
the morphological significance attaching to the inner membrane, 
which according to M. Sars is a vitelline membrane ? (3) Is 
M. Dohrn correct in comparing the membrane which he calls 
Larvenhaut in Asellus with the larval membrane of Ligia ? It is 
to the consideration of these three problems that M. Van Beneden 
devotes his attention in the memoir before us. He goes into the 
details of the embryology of Asellus which bear on the point ; 
traces out the whole early development of the species ; and illus- 
trates it by four plates containing several well-drawn representa- 
tions of the egg and larva in different phases; and finally concludes 
by laying down the following propositions : — (1) The ovarian egg, 
at the time of deposition, consists of only one membrane, the 
chorion. (2) This membrane remains for some considerable period 
the sole membrane of the egg. (3) The membrane, which both 
MM. Sars and Dohrn associate with the ovum, is really an 
embryonic membrane. (4) The study of the inferior Crustacea, 
and especially of Anchorellae and LerneopodaD, demonstrates that 
in these Lernese the embryo undergoes three moults even while in 
the egg: a Blastodermic moult, a Nauplian moult, and a Cyclopean 
or Zoean moult. That each of these moults consists in the loss of 
a cuticular membrane, having the form which the embryo has at 
the time the membrane is formed. The first embryonic membrane 
of Asellus is a Blastodermic cuticle, and subsequently there is 
formed a Nauplian cuticle which corresponds to the larval mem- 
brane of Ligia. M. Van Beneden's memoir is one which must be 
carefully studied by every student of Embryology. 
PEOGKESS OF MICKOSCOPICAL SCIENCE. 
The Development of the Choetopoda. — Under this title a very valu- 
able memoir has been written jointly by Professor Claparede and 
Mecznikow, in the last number of Siebold and KoUiker's Zeitschrift. 
It is illustrated by a great number (upwards of a hundred) beautifully- 
drawn coloured figures, and traces the stages of development in the 
typical members of the family of this group. Each species selected is 
dealt with separately and completely, and the whole memoir is one 
of the fullest published on the subject. 
The Anatomy of the Bed-hug. — The structure of Cimex lactularius 
is very fully described in another paper in Siebold's Zeitschrift by Herr 
Professor Landois. This is a continuation of a paper published some 
time since in this Journal by the same author. In this part Dr. 
Landois treats of the respiratory, reproductive, and muscular systems. 
VOL. II. Q 
