THE COMMON PLAN OF ANIMALS. 285 
gastrula, as the early stages of development. The like 
is true of all the worms, sea-urchins, starfishes, jellyfishes, 
polypes, and sponges; and it is only in the minutest and 
simplest forms of animal life that the germ, or repre- 
sentative of the ovum becomes metamorphosed into the 
adult form without the preliminary process of division. 
In the majority even of these Protozoa, the typical 
structure of the nucleated cell is retained, and the whole 
animal is the equivalent of a histological unit of one of 
the higher organisms. An Ameba is strictly comparable, 
morphologically, to one of the corpuscles of the blood of 
the crayfish. 
Thus, to exactly the same extent as it is legitimate 
to represent all the crayfishes as modifications of the 
common astacine plan, it is legitimate to represent all 
the multicellular animals as modifications of the gastrula, 
and the gastrula itself as a peculiarly disposed aggregate 
of cells; while the Protozoa are such cells either isolated, 
or otherwise aggregated. 
It is easy to demonstrate that all plants are either 
cell aggregates, or simple cells; and as it is impossible 
to draw any precise line of demarcation, either physio- 
logical or morphological, between the simplest plants, 
and the simplest of the Protozoa, it follows that all forms 
of life are morphologically related to one another; and 
that in whatever sense we say that the English and the 
Californian crayfish are allied, in the same sense, though 
not to the same degree, must we admit that all living things 
