142 
PROGRESS OF MICROSCOPICAL SCIENCE. 
National Academy of Sciences (U.S.A.), and appears in the * American 
Naturalist' (July, 1875). Mr. Packard observes : After a good many 
unsuccessful attempts at discovering the first indications of the 
nervous system in the embryo of Limulus, I at length, in making fine 
sections, with the aid of the skill of Prof. T. D. Biscoe, discovered 
it in a transverse section of an embryo in an early stage of develop- 
ment, corresponding to that figured on plate iv., fig. 10, of my essay 
on the Development of Limulus Polyphemus, in the ' Memoirs of the 
Boston Society of Natural History.' The period at which it was 
first observable was posterior to the first blastodermic moult, and 
before the appearance of the rudiments of the limbs. The primitive 
band now surrounds the yolk, being much thicker on one side of 
the egg than the other, the limbs budding out from this disk-like 
thickened portion which represents the outer or nervous layer of the 
germ. At the time the nervous cord was observed it was entirely 
differentiated from the nervous layer proper, and in section and rela- 
tion to the nervous layer appeared much as in Kpwalevsky's figure (33) 
of the germ of Hydrophilus.* 
At a later stage in the embryo (represented by pi. v., fig. 16, 
in my memoir), at a period when the body is divided into a head 
and abdomen, and the limbs are longer than before, by a series of 
sections parallel with the under surface of the body, I could make 
out quite satisfactorily the general form of the main nervous cord. 
It then forms a broad thick mass, the two cords being united with 
small holes between the cords opposite the sutures between the 
segments, and situated between the primitive ganglionic centres. 
The nervous cord, as in the Acarina, is formed long before the other 
internal systems of organs ; the development of the dorsal vessel 
some time after succeeding that of the nervous cord, while the 
alimentary canal is not formed until some time after the larva is 
hatched. 
The next stage observed, and one of exceeding interest, was 
studied in longitudinal sections of the larval Limulus. If we make 
a longitudinal section of the young king crab when a little over an 
inch long, the disposition of the cephalothoracic portion of the cord 
is exactly as in the full-grown individuals. The nervous gangh'a are 
then united into a continuous nervous collar surrounding the oeso- 
phagus, no ganglionic enlargements being observed, the collar in fact 
consisting entirely of ganglia, the commissures being obsolete ; in 
front of the oesophagus and in the same plane as the oesophageal collar 
lies the supraoesophageal ganglion, or so-called brain ; not as usual 
in the normal Crustacea, raised above the mouth into the roof of the 
head. On the contrary, the oesophagus passes behind the brain and 
through the collar at a right angle to the plane of the oesophageal 
collar and brain taken collectively. Now a section of the larva 
before moulting shows a most important and interesting difference as 
regards the ganglia which supply nerves to the appendages of the 
ccphalothorax. These are entirely separate, the spaces between 
them, where they are connected by commissures, being as wide as 
* ' Kmbryologische Studicn an Wurmen und Arthroj)odcn,' 1871. 
