50 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
and a stomach. The embryo becomes segmented into a head and four 
thoracic segments; on each side four coelom pockets arise from the 
gut ; the head-coelom is divided ventrally into paired sacs — the rudi- 
ments of the first pair of appendages ; the coelom pouches of the second 
and fourth thoracic segments give rise to appendages, while from the 
coelom pouches of the third thoracic segments there also arise gonads 
and a pair of mid-gut glands of doubtful nature. The oral papilla arises 
somewhat late from a proliferation of ectoderm, from which the teeth are 
also formed ; the salivary glands arise as ectodermic invaginations ; the 
muscles arise from the coelom pouches, and so do the glandular cells of 
the appendages. The four pairs of ganglia in the ventral chain and the 
paired sub-oesophageal ganglion are late in being separated from the 
ectoderm. Dorsally a paired ectodermic proliferation appears, which 
soon divides into three portions on each side ; the most ventral forms 
the brain, the median the optic ganglion, the most dorsal the eye. The 
eye is somewhat complex; each visual cell is surrounded by several 
pigment cells ; there is a single lens. Erlanger supposes that the head 
and the first two thoracic segments represent a cephalothorax ; that the 
third and fourth segments form an abdomen, behind which is a rudi- 
mentary post-abdomen or tail which degenerates. 
e. Crustacea. 
Development of Crustacea.* — Prof. L. Eoule has made a study of 
the development of Porcellio scaber. He finds that the fertilized egg 
diminishes in volume as it becomes converted into an embryo ; as a space 
becomes developed between the embryo and the egg-membrane it becomes 
filled with a liquid which plays the part of a protective cushion. The 
egg contains a large quantity of deutolecithin and several superficial 
islets of blastolecithin ; these two substances differ only in the presence 
or absence of vitelline granules, but the latter only gives rise to the cells 
of the embryo, while the former serves for food, being absorbed by 
phagocytosis. 
Segmentation is partial. As the blastoderm increases in extent, it 
divides into an outer epithelial layer, and an internal mass of cells 
scattered in the deutolecithin ; the former is the protectoderm, the 
latter the protendoderm ; the latter, again, undergoes division into a 
mesenchymatous part which is the mesoderm, and an epithelial endo- 
derm. The author points out that, from their origin and nature, the 
layers of Arthropods are not the homologues of the corresponding parts 
in other Coelomata. 
The so-called dorsal organ is really only a temporary dorsal swelling 
formed by deutolecithin which is not yet absorbed. The foundations of 
the nervous centres are simple, continuous, and unpaired. After absorp- 
tion of the deutolecithin most of the elements of the mesoderm are 
converted into connective cells, or by the formation of a sheath of sarco- 
plasm into muscular fibres. 
“ The irrigating apparatus ” is organized into a “ polycoelom,” which 
is likewise developed from the mesoderm ; its cavities correspond to more 
or less regular spaces hollowed out in the connective and muscular 
* Ann. Sci. Nat., xviii. (1894) pp. 1-156 (10 pis.). 
