FATE OF THE GERM-LAYERS. 87 



adult is obvious. It has an elongated, jointed body, traversed 

 by the alimentary canal, which opens in front by the mouth and 

 behind by the anus. The metamerism is expressed externally 

 by the jointed appearance, internally by the presence of paired 

 cavities (coalom) separated by dissepiments. Both the body-wall 

 and the alimentary wall consist of two layers : the former of 

 ectoblast without and somatic mesoblast within; the latter of 

 splanchnic mesoblast without (i.e., towards the body-cavity), 

 and either entoblast or ectoblast within, according as we con- 

 sider the mid-gut on the one hand, or the fore- and hind-gut on 

 the other. This is shown in Fig. 38, which represents a cross- 

 section of the embryo through the mid-gut. If this be clearly 

 l>orne in mind the development of all the other organs is easy to 

 understand, since they are formed as thickenings, outgrowths, 

 etc., of the parts already existing. For instance, the blood- 

 vessels make their appearance everywhere throughout the meso- 

 tlast, and the reproductive organs are at first mere thickenings 

 on the somatic layer of the mesoblast, afterwards separating 

 more or less from it so as to lie in the cavity of the coelom. 

 The nervous system is produced by thickenings and ingrowths 

 from the ectoblast. The origin of the different parts is shown 

 in the following scheme : 



THE GERM-LAYERS AND THEIR DERIVATIVES. 



The above statements * as to the origin of the various organs 

 acquire great interest in view of the fact that they are essen- 



* The nephridia have been omitted since their precise origin is in dispute. 

 It is certain that the outer portion of the tube (muscular part) is an ingrowth 

 from the ectoblast. The latest researches seem to show that the entire ne- 

 phridium has the same origin, though some authors describe the inner portion 

 as arising from mesoblast. 



