1889.3 Emhryo'o,;}:. 1019 
In the deevlopment of the nervous syst.ivi il^.cie a-c formed transitory 
giant ganglion cells which are shut out of tlie central nervous system 
and persist for a long time lying outside the cord. They apparently 
form a transitory larval nervous system, possibly analogous to the sub- 
umbrellar cells described by Kleinenberg as ushering in the permanent 
ventral cord in Lopadorhynchus. 
In the Verhandlung of the third meeting of the German Anatomical 
Society, Karl Bardeleben presented evidence for the existence of a 
sixth normal toe in the Mammalia. He finds in the skeletons of 
several forms bones on the radial side of the hand which he regards as 
evidence of a finger outside of the thumb, to which he gives the name 
prepollex ; the corresponding structure in the foot is the prehallux. 
The existence of these additional digits has been seriously questioned, 
the bones being regarded as sesamoid. In Pedetes capensis, however, 
Bardeleben finds a true sixth finger which is furnished with a nail, and 
which seems to represent a thumb in function. Tornier, at the meet- 
ing, regarded these sixth fingers and toes in the Mammalia as physio- 
logically new structures, not as ancestral features. 
EMBRYOLOGY. 
Evolution of the Medullary Canal.— Under this head we have 
to consider, first, what is the primitive vertebrate type of the central 
nervous system ; second, what genetic relation existed between the ver- 
tebrate and invertebrate types. 
The "opinion generally accepted by embryologists is that the typical 
vertebrate canal is formed by the closure of the medullary groove. 
This view is advocated by Balfour, and has been so thoroughly 
accepted by Adam Sedgwick that he has made it the basis of a 
speculation! on the original function of the canal; he supposes 
that it was open behind and excretory : the cilia which are found in 
the central canal of the spinal cord originally served to produce the 
excretory current. This opinion overlooks the serious diflliculty of 
assuming that the canal is primitive, while in the lowest vertebrates it 
is clearly a secondary modification. In Petromyzon, Lepidosteus 
and Teleosts, the medullary plate, instead of becoming the floor of an 
external groove, forms a solid keel-like projection towards the ventral 
of the Central Nervous System 
, 325-328. 
