42 SCIENCE AND PHILOSOPHY OF THE ORGANISM 



cell-masses of the mesen chyme ; 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 



