1892, ] Embryology. Til 
ceral ganglion arises from a single thickening of the wall of the man- 
tle chamber near the pericardium. 
There is no “Scheitelplatte,” the two rudiments of the cerebral 
ganglia being from the first distinct. They arise by delamination from 
two distinct thickenings of the ectoderm of the pre-velar area, situated 
one on the right, one on the left of the centre of the area. 
The pedal ganglia next appear, arising in the same way by delami- 
nation from two distinct ectodermal thickenings. The pallial ganglia 
develop from similar paired rudiments. The buccal ganglia are formed 
later from paired thickenings of the ectoderm of the ventral wall of 
the stomodeum. Two thickenings of the anterior border of the man- 
tle give rise to the two intestinal ganglia, which are at first right and 
left. The twisting of the embryo soon brings the right one above the 
intestine (supraintestinal ganglion) and the left one below (subintesti- 
nal ganglion). The subintestinal ganglion does not appear in the 
adult. The visceral ganglion is formed from the posterior part of the 
floor of the mantle chamber. It arises from the ectoderm, but does 
not have a paired origin, differing in this respect from all the other 
ganglia. 
As seen in the order of description, the ganglia arises progressively 
from before backward. e commissures and connectives arise in the 
same order. The cerebral ganglia first connect with each other, then 
with the pallial, the pedal and the buccal ganglia. The pedal ganglia 
next unite and then the buccal ganglia. No commissure connects the 
pallial ganglia. Of all the connectives between the cerebral ganglia 
and the other nerve centres the cerebro-pedal connectives are the last 
to appear. This is the only exception to the rule that the nervous 
system develops progressively from before backward. Because of the 
small size of the ganglion cells Erlanger was unable to demonstrate 
the origin of the nerve fibres. 
“ The circulatory system of Paludina arises in the manner typical 
for the Mollusca.” 
The origin of the sexual organs is interesting. It is the same in both 
sexes. The ovary, or testis, is formed from an evagination of a por- 
tion of the pericardium, almost, or exactly, in the place where earlier 
the rudimentary left kidney was formed. The duct arises as did the 
rudimentary left renal duct and from the same region of the mantle 
chamber. The tubular rudiment of the “sexual gland” separates 
from the pericardium and forms a hollow vesicle, which, later, con- 
nects with the sexual duct. The sexual organs are formed, then, 
apparently by the reopening of the left kidney, which appeared and 
