While Cuvier and his followers traced these four distinct plans, as shown in the adult animal, Baer opened to us a new field of investigation in the embryology of the four types, showing that for each there was a special mode of growth in the egg.  Looking at them from this point of view, we shall see that these four types, with their four modes of growth, seem to fill out completely the plan or outline of the animal kingdom, and leave no reason to expect any further development or any other plan of animal life within these limits.  The eggs of all animals are spheres, such as I have described them; but in the Radiate the whole periphery is transformed into the germ, so that it becomes, by the liquefying of the yolk, a hollow sphere.  In the Mollusks, the germ lies above the yolk, absorbing its whole substance through the under side, thus forming a massive close body instead of a hollow one.  In the Articulate, the germ is turned in a position exactly opposite to that of the Mollusk, and absorbs the yolk upon the back.  In the Vertebrate, the germ divides in two folds, one turning upward, the other turning downward, above and below the central backbone.  These four modes of development seem to exhaust the possibilities of the primitive sphere, which is the foundation of all animal life, and therefore I believe that Cuvier and Baer were right in saying that the whole animal kingdom is included under these four structural ideas.

Leuckart proposed to subdivide the Radiates into two groups:  the Coelenterata, including Polyps and Acalephs or Jelly-Fishes,—­and Echinoderms, including Star-Fishes, Sea-Urchins, and Holothurians.  His reason for this distinction is the fact that in the latter the organs are inclosed within walls of their own, distinct from the body-wall; whereas in the former the organs are formed by internal folds of the outer wall of the body, as in the Polyps, or are hollowed out of the substance of the body, as in Jelly-Fishes.  This implies no difference in the plan, but merely a difference in the execution of the plan.  Both are equally radiate in their structure; and when Leuckart separated them as distinct primary types, he mistook a difference in the material expression of the plan for a difference in the plan itself.  So some naturalists have distinguished Worms from the other Articulates as a separate division.  But the structural plan of this type is a body divided by transverse constrictions or joints; and whether those joints are uniformly arranged from one end of the body to the other, as in the Worms, or whether the front joints are soldered together so as to form two regions of the body, as in Crustacea, or divided so as to form three regions of the body, as in winged Insects, does not in the least affect the typical character of the structure, which remains the same in all.  Branches or types, then, are natural groups of the animal kingdom, founded on plans of structure or structural ideas.