ELEMENTARY MORPHOGENESIS 41 



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 5; 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 endo- 

 derm has acquired bilateral symmetry just as the mesen- 

 chyme 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 organo- 

 genesis 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 



