218 
8UMMARY OF CURRENT RESEARCHES RELATING TO 
connected by their processes and separated by fluid. Cilia and pigment 
next appeared on the outer cells, except at one pole. 
After describing the metamorphosis the author points out that the 
history of development favours Metschnikotfs theory that the ancestors 
of Spionges were solid. Attention is drawn to the striking resemblance 
between this asexual development and the egg-development of many 
siliceous Sponges. This resemblance can, the author thinks, only be 
explained on the supposition that there is some essential likeness 
between the me6odcrm-cells which make up the gemmule and ova. He 
is inclined to regard the gemmule cell as a true germ-cell in which all 
the genu-plasm remains undifferentiated, or, in other words, is not 
transformed into oogenetic phism. Moreover, the gemmule-cell preserves 
the parthenogenetic course of development — it keeps all its germ-plasm. 
As gemmules apparently develope anywhere in the Sponge-mesenchyme 
it must be supposed that any mesenchyme-cell may become a gemmule- 
cell, and consequently that it contains germ-plasm. We are led to the 
same conclusion by the study of egg-development, for it seems that any 
mesenchyme-cell may develope into an egg. 
The gemmule development in Tcdania is of much the same character 
as in Esperella . Tcdania and Hircinia have both solid morulse. 
Histology of Leucosolenia.* — Mr. E. A. Minchin gives an account 
of a sieve-like membrane which he has observed across the oseula of a 
species of Leucosolenia, together with some observations on the histology 
of that Sponge. This membrane is composed of two layers of cells in 
aprposition, but separated by a thin layer of jelly ; the cells are continued 
out from their central portion into three, four, five, or even six processes, 
which, uniting with the processes of other cells, form a network with 
comparatively wide meshes. The best preparations are obtained by 
fixing with osmic acid, which preserves the shape of the network, and 
staining with pnCrocarmine, which removes the blackening to a great 
extent. The inner of the two layers of the membrane becomes directly 
continuous with the layer of collared epithelium which composes the 
endoderm of the sponge. The outer layer is similarly continuous with 
the ectoderm. It is probable that the inner layer is of endodt rmal, and 
the outer of ectodermal origin. The function of this membrane appears 
to be that of keeping intruding animals out of the gastric cavity. 
The collar- cells of the endoderm were observed to vary in shape, 
and to have no trace of a collar, which, therefore, must be supposed to 
be retractile. The flagellum is twice as high as the cell or higher ; it 
makes a somewhat slow stroke to the right, followed immediately by a 
quick stroke to the left ; after a pause these motions are repeated. The 
contact of a foreign body stimulates a flagellum to greater activity. 
The process is really cylindrical, and the aprpareut tapering to a point 
is an optical delusion. 
No trace of muscular, elastic or other special cells was found in the 
mesoderm. The superficial ectoderm cells were very difficult to make out, 
and the author suggests that, in the fully-formed sponge, the ectoderm- 
cells may, to a certain extent, degenerate into a cuticle-like structure. 
No trace in either living or preserved specimens could be detected of 
* Quart. Journ. Slier. £ci., xxxiii. (1S92) pp. 251-72 (2 pis.). 
