Agassiz— Paleontological and Embryological Development. 297 
any organ and its subsequent modifications, they throw but 
little hight on the subject before us. What we need for our 
comparisons are the various stages of growth through which 
the young Sea-urchins of different families pass from the time 
they have practically become Sea-urchins until they have 
attained the stage which we now dignify with the name of 
species. Few embryologists have carried their investigations 
into the more extended field of the changes the embryo under- 
goes when it begins to be recognized as belonging to a special 
class, and when the knowledge of the specialist is absolutely 
needed to trace the bearing of the changes undergone, and to 
understand their full meaning. Fortunately the growth of the 
young Kchini has been traced in a sufficient number of fami- 
lies to enable me to draw the parallelism between these various 
stages of growth and the paleontological stages in a very differ- 
ent manner from what is possible in other groups of the animal 
kingdom, where we are overwhelmed with the number of spe- 
cies, as in the Insects or Mollusks, or where the paleontological 
or the embryological terms of comparison are wanting or ver 
imperfect. 
ne of these, the genus Cidaris, has continued to exist, with 
(Temnocidaris) so marked at the present day in the genus 
