40 SCIENCE AND PHILOSOPHY OF THE ORGANISM 



other. A figure shows best what has occurred, and you will 

 notice (Fig. 3) two masses of cells in this figure, which 

 have the forms of spherical triangles : it is in the midst of 

 these triangles that the skeleton of the larva originates. 

 The germ had an upper and a lower side before : it now 

 has got an upper and lower, front and back, right and left 

 half; it now has acquired that symmetry of organisation 



o 



a 



o 



o 



o 



o 



O' 



o o a 



o 



o 



FIG. 3. FORMATION OF MESENCHYME IN ECHINUS. 



. Outlines of blastula, side-view; mesenchyme forms a heap of cells at the "vegeta- 

 tive " pole. 



cij. Heap of inesenchyme-cells from above. 



b. Mesenchyme-cells arranged in a ring round the vegetative pole. 



<. Mesenchyme-cells arranged in a bilateral-symmetrical figure ; primordia of skeleton in 

 the midst of two spherical triangles. 



which our own body has ; at least it has got it as far as 

 its mesenchyme is concerned. 



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 cells 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 



