GANGLIA AND NERVES OF SQUALUS 21 
cluded in the body ot the paper. The general morphological 
relations of ganglia are fairly safe criteria for their identifica- 
tion provided one is sure of their presence; otherwise, the dis- 
tribution of the fibers must be known to identify a ganglion with 
certainty. 
The differences in size in the cells of different ganglionic com- 
ponents sometimes found in other types and particularly in 
mature types do not seem to exist in the material studied; such 
differences as exist are those between older and younger gan- 
glion cells rather than between different components. How- 
ever, I do not believe any serious oversight has been made 
unless it is in the failure to find a general cutaneous component 
in the seventh and ninth nerves. 
The number of nerves described is small compared with the 
adult, of course, but those present in the stage described are the 
chief nerves. While this study should have followed and not 
preceded that of older material, it is hoped that it will help to 
fill the gap in our knowledge of our most generalized vertebrate 
and it will certainly serve as a foundation for the study of the 
origin of the cerebral] ganglia on a component basis. The effort 
to describe the origin of these ganglia, en masse, is entirely futile 
in the author’s opinion. It must be done with a thorough 
knowledge of all the ganglia involved. This is true whether 
they arise as discrete ganglia having different sources of origin 
or whether they differentiate out of a common primordium. 
The author is under obligation to Dr. H. V. Neal for most of 
the material, which was fixed in vom Rath’s fluid and mounted 
unstained. This material was supplemented by younger ma- 
terial stained in Delafield’s haematoxylin and orange G. The 
embryos ranged in length from 18 to 30 mm. after fixation and 
several specimens of each length, except the 30 mm. embryo, 
were examined. The plot and drawings were made from a 22 
mm. embryo. The terms anterior, posterior, dorsal ,and ven- 
tral are used in the body of the paper to indicate the relative 
positions of structures on the plot and not their true position 
in the adult which is sometimes quite different. 
