SCIENTIFIC SUMMARY. 
135 
organ antagonistic to the muscular layer. The second kind of connective 
tissue is a substance destitute of cells, which forms the umbrella of all the 
simple Medusae, including the gelatinous substance of the natatory bells and 
covering laminae of the Siphonophora. Sometimes this substance is entirely 
homogeneous, sometimes it is traversed by numerous fibres veiy like the 
elastic fibres. In an iEquorea these fibres are attached to a membrane capable 
of isolation, placed beneath the epithelium of the convex surface of the um- 
brella. The third form is the well-known gelatinous substance, with dissemi- 
nated cells, of the umbrella of the higher Medusae. Professor Kolliker agrees 
with Professor Virchow in denying the existence of these cells in Cyancea 
capillata. — Vide The Quarterly Journal of Microscopical Science , No. XX. 
The Episternal Apparatus of Mammalia. — Herr Gegenbaur has published 
a valuable essay upon the episternal bones of mammals, which has been 
translated in the Natural History Review (No. XX.). He considers that 
there are three principal varieties in the conformation which these bones 
assume in quadrupeds. The first, which may be regarded as the most com- 
plete, is that in which the episternal consists of a median piece connected 
with the sternum, and carrying two lateral transverse portions. The second 
form is characterized by the presence of the median piece alone, and is seen 
in the frog and crocodile. The third form is characterized by the absence of 
the median piece, the two lateral portions being still visible. The whole 
structure seems here to be dependent upon the presence of a clavicle, and 
from its appearance, with few modifications, it constitutes a connecting medium 
between the clavicle and the sternum. — Vide the Jenaische Zeitschrift fur 
Medecin, vol. I. p. 175, and Natural History Review , as above. 
The Transformations of Chloeon dimidiatum have been most carefully 
observed and detailed in a memoir written by Sir John Lubbock, F.R.S. 
This insect is one of the Ephemeridse or day-flies. The larva, in the earliest 
stage observed by Sir John Lubbock, is a minute, transparent, active creature, 
with a large head, a tapering abdomen, and two long caudal appendages. It 
spends about a year in the water, during which time it increases in size, and 
alters considerably in appearance. The changes, however, are produced quite 
gradually, the insect going through about twenty changes of form, each ac- 
companied by a change of skin. The antennae increase in length and 
in the number of segments at each moult, and it is remarkable that 
this increase is not produced by a growth of the entire organ, but by 
a rapid development and division of the third segment, counting from 
the base. In the first stage, the larva has no respiratory organs, either 
external or internal. After the first change of skin, however, the pos- 
terior angles of several abdominal segments become elongated, and after 
one or two more moults, these elongations have taken the form of the 
gills or branchiae characteristic of the species. At the same time, the tracheae 
make their appearance. So far as the author is aware, no other insect has 
yet been observed which is entirely distitute of tracheae. After the first one or 
two moults, a minute knob appears between the two caudal appendages, and 
with each moult this knob increases in length, so that the larva, which had 
originally two tails, finally possesses three. After about eight moults have 
taken place, it may be observed that the posterior mesothoracic angles are 
slightly elongated. At each change of skin, these, the first rudiments of the 
