We leave the mesenchyme for a while and study another kind of organogenesis. At the very same pole of the germ where the mesenchyme cells originated there is a long and narrow tube of cell growing in, and this tube, getting longer and longer, after a few hours of growth touches the opposite pole of the larva. The growth of this cellular tube marks the beginning of the formation of the intestine, with all that is to be derived from it. The larva now is no longer a blastula, but receives the name of “gastrula” in Haeckel’s terminology; it is built up of the three “germ-layers” in this stage. The remaining part of the blastoderm is called “ectoderm,” or outer layer; the newly-formed tube, “endoderm,” or inner layer; while the third layer is the “mesenchyme” already known to us.
The endoderm itself is a radial structure at first, as was the whole germ in a former stage, but soon its free end bends and moves against one of the sides of the ectoderm, against that side of it where the two triangles of the mesenchyme are to be found also. Thus the endoderm has acquired bilateral symmetry just as the mesenchyme before, and as in this stage the ectoderm also assumes a bilateral symmetry in its form, corresponding with the symmetrical relations in the endoderm and the mesenchyme, we now may call the whole of our larva a bilateral-symmetrical organisation.
It cannot be our task to follow all the points of organogenesis of Echinus in detail. It must suffice to state briefly that ere long a second portion of the mesenchyme is formed in the larva, starting from the free end of its intestine tube; that the formation of the so-called “coelum” occurs by a sort of splitting off from this same original organ; and that the intestine itself is divided into three parts of different size and aspect by two circular sections.
But we must not, I think, dismiss the formation of the skeleton so quickly. I told you already that the skeleton has its first origin in the midst of the two triangular cell-masses of the mesenchyme; but what are the steps before it attains its typical and complicated structure? At the beginning a very small tetrahedron, consisting of carbonate of calcium, is formed in each of the triangles; the four edges of the tetrahedron are produced into thin rods, and by means of a different organogenesis along each of these rods the typical formation of the skeleton proceeds. But the manner in which it is carried out is very strange and peculiar. About thirty of the mesenchyme cells are occupied in the formation of skeleton substance on each side of the larva. They wander through the interior space of the gastrula—which at this stage is not filled with sea water but with a sort of gelatinous material—and wander in such a manner that they always come to the right places, where a part of the skeleton is to be formed; they form it by a process of secretion, quite unknown in detail; one of them forms one part, one the other, but what they form altogether, is one whole.
When the formation of the skeleton is accomplished, the typical larva of our Echinus is built up; it is called the “pluteus” (Fig. 4). Though it is far from being the perfect adult animal, it has an independent life of its own; it feeds and moves about and does not go through any important changes of form for weeks. But after a certain period of this species of independent life as a “larva,” the changes of form it undergoes again are most fundamental: it must be transformed into the adult sea-urchin, as all of you know. There are hundreds and hundreds of single operations of organogenesis to be accomplished before that end is reached; and perhaps the strangest of all these operations is a certain sort of growth, by which the symmetry of the animal, at least in certain of its parts—not in all of them—is changed again from bilateral to radial, just the opposite of what happened in the very early stages.
Fig. 4.—Larval Development of Echinus.
| A. | The gastrula. |
| B. | Later stage, bilateral-symmetrical. Intestine begins to divide into three parts. |
| C. | Pluteus larva. S = Skeleton. I = Intestine. |
But we cannot follow the embryology of our Echinus further here; and indeed we are the less obliged to do so, since in all our experimental work we shall have to deal with it only as far as to the pluteus larva. It is impossible under ordinary conditions to rear the germs up to the adult stages in captivity.
You now, I hope, will have a general idea at least of the processes of which the individual development of an animal consists. Of course the specific features leading from the egg to the adult are different in each specific case, and, in order to make this point as clear as possible, I shall now add to our description a few words about what may be called a comparative descriptive embryology.